Melatonin attenuates the sympathetic nerve responses to orthostatic stress in humans

Melatonin and Vascular Function

The indolamine hormone melatonin, also known as N-acetyl-5-methoxytrypamine, is frequently associated with circadian rhythm regulation. Light can suppress melatonin secretion, and photoperiod regulates melatonin levels by promoting its production and secretion at night in response to darkness. This hormone is becoming more and more understood for its functions as an immune-modulatory, anti-inflammatory, and antioxidant hormone. Melatonin may have a major effect on several diabetes-related disturbances, such as hormonal imbalances, oxidative stress, sleep disturbances, and mood disorders, according to recent research. This has raised interest in investigating the possible therapeutic advantages of melatonin in the treatment of diabetic complications. In addition, several studies have described that melatonin has been linked to the development of diabetes, cancer, Alzheimer's disease, immune system disorders, and heart diseases. In this review, we will highlight some of the functions of melatonin regarding vascular biology.

The Role and Therapeutic Potential of Melatonin in Degenerative Fundus Diseases: Diabetes Retinopathy and Age-Related Macular Degeneration

Degenerative fundus disease encompasses a spectrum of ocular diseases, including diabetic retinopathy (DR) and age-related macular degeneration (AMD), which are major contributors to visual impairment and blindness worldwide. The development and implementation of effective strategies for managing and preventing the onset and progression of these diseases are crucial for preserving patients' visual acuity. Melatonin, a neurohormone primarily produced by the pineal gland, exhibits properties such as circadian rhythm modulation, antioxidant activity, anti-inflammatory effects, and neuroprotection within the ocular environment. Furthermore, melatonin has been shown to suppress neovascularization and reduce vascular leakage, both of which are critical in the pathogenesis of degenerative fundus lesions. Consequently, melatonin emerges as a promising therapeutic candidate for degenerative ocular diseases. This review provides a comprehensive overview of melatonin synthesis, its localization within ocular tissues, and its mechanisms of action, particularly in regulating melatonin production, thereby underscoring its potential as a therapeutic agent for degenerative fundus diseases.

Melatonin supplementation reduces nighttime blood pressure but does not affect blood pressure reactivity in normotensive adults on a high-sodium diet

High-sodium diets (HSDs) can cause exaggerated increases in blood pressure (BP) during physiological perturbations that cause sympathetic activation, which is related to cardiovascular risk. Melatonin supplementation has been shown to play a role in BP regulation. Our aim was to examine the effects of melatonin taken during an HSD on 24-h BP and BP reactivity during isometric handgrip (IHG) exercise, postexercise ischemia (PEI), and the cold pressor test (CPT). Twenty-two participants (11 men/11 women, 26.5 ± 3.1 yr, BMI: 24.1 ± 1.8 kg/m2, BP: 111 ± 9/67 ± 7 mmHg) were randomized to a 10-day HSD (6,900 mg sodium/day) that was supplemented with either 10 mg/day of melatonin (HSD + MEL) or placebo (HSD + PL). Twenty-four-hour ambulatory BP monitoring was assessed starting on day 9. Mean arterial pressure (MAP) was quantified during the last 30 s of IHG at 40% of maximal voluntary contraction and CPT, and during 3 min of PEI. Melatonin did not change 24-h MAP (HSD + PL: 83 ± 6 mmHg; HSD + MEL: 82 ± 5 mmHg; P = 0.23) but decreased nighttime peripheral (HSD + PL: 105 ± 10 mmHg; HSD + MEL: 100 ± 10 mmHg; P = 0.01) and central systolic BP (HSD + PL: 97 ± 9 mmHg; HSD + MEL: 93 ± 8 mmHg; P = 0.04) on the HSD compared with the HSD + PL. The absolute and percent change in MAP during IHG was not different between conditions (all P > 0.05). In conclusion, melatonin supplementation did not alter BP reactivity to the perturbations tested on an HSD but may be beneficial in lowering BP in young healthy normotensive adults.NEW & NOTEWORTHY BP reactivity was assessed during isometric handgrip (IHG) exercise, postexercise ischemia (PEI), and the cold pressor test (CPT) after 10 days of a high-sodium diet with and without melatonin supplementation. Melatonin did not alter BP reactivity in healthy normotensive men and women. However, melatonin did decrease nighttime peripheral and central systolic BP, suggesting it may be beneficial in lowering BP even in those with a normal BP.

Associations between actigraphy-derived rest-activity rhythm characteristics and hypertension in United States adults

People with disrupted circadian rhythms, such as shift workers, have shown a higher risk of hypertension. However, it is unclear whether more subtle differences in diurnal rest-activity rhythms in the population are associated with hypertension. Clarifying the association between the rest-activity rhythm, a modifiable behavioural factor, and hypertension could provide insight into preventing hypertension and possibly cardiovascular diseases. In this study, we investigated the association between rest-activity rhythm characteristics and hypertension in a large representative sample of United States adults. Cross-sectional data were obtained from the National Health and Nutrition Examination Survey 2011-2014 (N = 6726; mean [range] age 49 [20-79] years; 52% women). Five rest-activity rhythm parameters (i.e., pseudo F statistic, amplitude, mesor, amplitude:mesor ratio, and acrophase) were derived from 24-h actigraphy data using the extended cosine model. We performed multiple logistic regression to assess the associations between the rest-activity rhythm parameters and hypertension. Subgroup analysis stratified by age, gender, race/ethnicity, body mass index and diabetes status was also conducted. A weakened overall rest-activity rhythm, characterised by a lower F statistic, was associated with higher odds of hypertension (odds ratio quintile 1 versus quintile 5 [OR Q1vs.Q5 ] 1.61, 95% confidence interval [CI] 1.26-2.05; p trend < 0.001). Similar results were found for lower amplitude (OR Q1vs.Q5 1.51, 95% CI 1.13-2.03; p trend = 0.01) and amplitude:mesor ratio (OR Q1vs.Q5 1.34, 95% CI 1.01-1.78; p trend = 0.03). The results were robust to the adjustment of confounders, individual behaviours including physical activity levels and sleep duration and appeared consistent across subgroups. Possible interaction between the rest-activity rhythm and body mass index was found. Our results support an association between weakened rest-activity rhythms and higher odds of hypertension.

Melatonin and the Brain–Heart Crosstalk in Neurocritically Ill Patients—From Molecular Action to Clinical Practice

Brain injury, especially traumatic brain injury (TBI), may induce severe dysfunction of extracerebral organs. Cardiac dysfunction associated with TBI is common and well known as the brain–heart crosstalk, which broadly refers to different cardiac disorders such as cardiac arrhythmias, ischemia, hemodynamic insufficiency, and sudden cardiac death, which corresponds to acute disorders of brain function. TBI-related cardiac dysfunction can both worsen the brain damage and increase the risk of death. TBI-related cardiac disorders have been mainly treated symptomatically. However, the analysis of pathomechanisms of TBI-related cardiac dysfunction has highlighted an important role of melatonin in the prevention and treatment of such disorders. Melatonin is a neurohormone released by the pineal gland. It plays a crucial role in the coordination of the circadian rhythm. Additionally, melatonin possesses strong anti-inflammatory, antioxidative, and antiapoptotic properties and can modulate sympathetic and parasympathetic activities. Melatonin has a protective effect not only on the brain, by attenuating its injury, but on extracranial organs, including the heart. The aim of this study was to analyze the molecular activity of melatonin in terms of TBI-related cardiac disorders. Our article describes the benefits resulting from using melatonin as an adjuvant in protection and treatment of brain injury-induced cardiac dysfunction.

Effect of Preoperative Administration of Oral Melatonin on Pneumatic Tourniquet-Induced Ischemia-Reperfusion Injury in Orthopedic Surgery of Lower Extremities: A Randomized Clinical Trial

Background

Ischemic reperfusion injury (IRI) causes cellular damage and dysfunction. The present study aimed to evaluate the effect of melatonin on pneumatic tourniquet-induced IRI in orthopedic surgery of the lower extremities.

Methods

A randomized clinical trial was conducted at Chamran Hospital, Shiraz University of Medical Sciences (Shiraz, Iran), from September to November 2019. The target population was patients scheduled for elective orthopedic surgery of the lower extremities. A total of 67 patients were randomly divided into two groups, placebo (n=34) and melatonin (n=33). The groups received 10 mg melatonin or placebo the night before surgery and two hours before surgery. Primary outcome variables were the serum levels of superoxide dismutase (SOD) and malondialdehyde (MDA). Hemodynamic parameters, sedation score, and drug side effects were also evaluated. Data were analyzed using SPSS version 21.0 software. P<0.05 was considered statistically significant.

Results

In the analysis phase, due to loss to follow-up (n=26), 41 patients divided into two groups of melatonin (n=20) and placebo (n=21) were evaluated. There was no significant difference in demographic data, duration of surgery (P=0.929), and tourniquet time (P=0.496) between the groups. The serum levels of SOD and MDA were not significantly different between the groups (P=0.866 and P=0.422, respectively), nor were the incidence of postoperative nausea (P=0.588) and patients' satisfaction (P=0.088). However, the postoperative sedation score and vomiting between the groups were significantly different (P<0.001).

Conclusion

Administration of 10 mg melatonin provided effective sedation, but had no significant effect on the serum levels of SOD and MDA, nor on pneumatic tourniquet-induced IRI in orthopedic surgery of the lower limbs.

Trial registration number

IRCT20141009019470N87.

What do we really know about the safety and efficacy of melatonin for sleep disorders?

Melatonin is a hormonal product of the pineal gland, a fact that is often forgotten. Instead it is promoted as a dietary supplement that will overcome insomnia, as an antioxidant and as a prescription only drug in most countries outside the United States of America and Canada. The aim of this review is to step back and highlight what we know about melatonin following its discovery 60 years ago. What is the role of endogenous melatonin; what does melatonin do to sleep, body temperature, circadian rhythms, the cardiovascular system, reproductive system, endocrine system and metabolism when administered to healthy subjects? When used as a drug/dietary supplement, what safety studies have been conducted? Can we really say melatonin is safe when it has not been systematically studied and many studies show interactions with a wide range of physiological processes? Finally the results of studies investigating the efficacy of melatonin as a drug to alleviate insomnia are critically evaluated. In summary, melatonin is an endogenous pineal gland hormone with specific physiological functions in animals and humans, with its primary role in humans to maintain synchrony of sleep with the day/night cycle. When administered as a drug it affects a wide range of physiological systems and has clinically important drug interactions. With respect to efficacy for treating sleep disorders, melatonin can advance the time of sleep onset but the effect is modest and variable. In children with neurodevelopmental disabilities melatonin appears to have the greatest impact on sleep onset but little effect on sleep efficiency.

Melatonin as an Antiepileptic Molecule: Therapeutic Implications via Neuroprotective and Inflammatory Mechanisms

Epilepsy is a result of unprovoked, uncontrollable, and repetitive outburst of abnormal and excessive electrical discharges, known as seizures, in the neurons. Epilepsy is a devastating neurological condition that affects 70 million people globally. Unfortunately, only two-thirds of epilepsy patients respond to antiepileptic drugs while others become drug resistant and may be more prone to epilepsy comorbidities such as SUDEP. Oxidative stress, mitochondrial dysfunction, imbalance in the excitatory and inhibitory neurotransmitters, and neuroinflammation are some of the common pathologies of neurological disorders and epilepsy. Studies suggests that melatonin, a pineal hormone that governs sleep-wake cycles, may be neuroprotective against neurological disorders and thus may be translated as an antiepileptic as well. Melatonin has been shown to be an antioxidant, antiexcitotoxic, and anti-inflammatory hormone/molecule in neurodegenerative diseases, which may contribute to its antiepileptic and neuroprotective properties in epilepsy as well. In addition, melatonin has evidently been shown to play a regulatory role in the cardiorespiratory system and sleep-wake cycles, which may have positive implications toward epilepsy associated comorbidities, such as SUDEP. However, studies investigating the changes in melatonin release due to epilepsy and melatonin's antiepileptic role have been inconclusive and scarce, respectively. Thus, this comprehensive review aims to summarize and elucidate the potential role of melatonin in the pathogenesis of epilepsy and its comorbidities, in hopes to develop new diagnostic and therapeutic approaches that will improve the lives of epileptic patients, particularly those who are drug resistant.

Melatonin Exerts Anti-Inflammatory, Antioxidant, and Neuromodulatory Effects That Could Potentially Be Useful in the Treatment of Vertigo

The acute phase of vertigo involves multiple neurotransmitters, inflammatory mediators, and products of oxidative stress. The vestibular pathway has multiple melatonin receptors distributed along its path, both centrally and peripherally. In addition, melatonin has been shown to be a powerful antioxidant and anti-inflammatory agent against factors related to vertigo, such as Bax/caspases, interleukins, and chemokines. Likewise, it exerts central GABAergic, antidopaminergic, and anti-migraine functions and regulates sympathetic activity in a similar way to the drugs classically used in acute vestibular crisis. In this review, the role of melatonin as a potential treatment of the acute phase of vertigo is discussed.

The use of melatonin to mitigate the adverse metabolic side effects of antipsychotics

Antipsychotic drugs are efficacious first-line treatments for many individuals diagnosed with a psychiatric illness. However, their adverse metabolic side-effect profile, which resembles the metabolic syndrome, represents a significant clinical problem that increases morbidity and limits treatment adherence. Moreover, the mechanisms involved in antipsychotic-induced adverse metabolic effects (AMEs) are unknown and mitigating strategies and interventions are limited. However, recent clinical trials show that nightly administration of exogenous melatonin may mitigate or even prevent antipsychotic-induced AMEs. This clinical evidence in combination with recent preclinical data implicate the circadian system in antipsychotic-induced AMEs and their mitigation. In this chapter, we provide an overview on the circadian system and its involvement in antipsychotic-induced AMEs, as well as the potential beneficial effect of nightly melatonin administration to mitigate them.

Aortic Arch Calcification on Routine Chest Radiography is Strongly and Independently Associated with Non-Dipper Blood Pressure Pattern

Background

Non-dipper blood pressure (NDBP) is one of the important causes of hypertension-related target organ damage and future cardiovascular events. Currently, there is no practical tool to predict NDBP pattern.

Objectives

The aim of this study was to investigate the relationship between aortic arch calcification (AAC) on chest radiography and NDBP pattern.

Methods

All patients referred for ambulatory BP monitoring test were approached for the study participation. NDBP was defined as the reduction of ≤10% in nighttime systolic BP as compared to the daytime values. AAC was evaluated with chest radiography and inter-observer agreement was analyzed by using kappa statistics. Univariate and multivariate logistic regression analysis was conducted to assess the association of AAC and NDBP pattern. A 2-tailed p-value < 0.05 was considered statistically significant.

Results

A total of 406 patients (median age: 51.3) were included. Of these, 261(64%) had NDBP pattern. Overall, the prevalence of AAC was 230 (57%). Non-dipper group had significantly higher prevalence of AAC (70% vs. 33%, p < 0.0001) as compared to the dipper group. Presence of AAC was a strong and independent predictor of NDBP pattern (OR 3.919, 95%CI 2.39 to 6.42) in multivariate analysis.

Conclusions

Presence of AAC on plain chest radiography is strongly and independently associated with the presence of NDBP pattern.

Melatonin Relations with Energy Metabolism as Possibly Involved in Fatal Mountain Road Traffic Accidents

Previous results evidenced acute exposure to high altitude (HA) weakening the relation between daily melatonin cycle and the respiratory quotient. This review deals with the threat extreme environments pose on body time order, particularly concerning energy metabolism. Working at HA, at poles, or in space challenge our ancestral inborn body timing system. This conflict may also mark many aspects of our current lifestyle, involving shift work, rapid time zone crossing, and even prolonged office work in closed buildings. Misalignments between external and internal rhythms, in the short term, traduce into risk of mental and physical performance shortfalls, mood changes, quarrels, drug and alcohol abuse, failure to accomplish with the mission and, finally, high rates of fatal accidents. Relations of melatonin with energy metabolism being altered under a condition of hypoxia focused our attention on interactions of the indoleamine with redox state, as well as, with autonomic regulations. Individual tolerance/susceptibility to such interactions may hint at adequately dealing with body timing disorders under extreme conditions.

Salt Loading Aggravates the Relationship between Melatonin and Proteinuria in Patients with Chronic Kidney Disease

Objective Salt loading induces renal damage independently of blood pressure (BP) elevation via reactive oxygen species and sympathetic activity. Melatonin, a hormone that regulates the circadian rhythm, has multiple functions, including anti-oxidant effects and the inhibition of sympathetic activity. We have shown that impaired melatonin secretion is associated with renal damage in chronic kidney disease (CKD) patients. However, the associations between salt loading, melatonin secretion, and urinary albumin and protein have not been clarified. Methods We recruited 32 CKD patients, conducted 24-hour ambulatory BP monitoring and collected daytime and nighttime urine while the patients were consuming a standard salt (10 g/day) or low salt (6 g/day) diet. The excretion levels of albumin, protein and 6-sulfatoxymelatonin (aMT6s), a metabolite of melatonin, in daytime and nighttime urine were investigated in patients consuming standard salt and low salt diets. Results The urinary aMT6s levels in daytime and nighttime of the patients consuming standard salt and low salt diets did not differ to a statistically significant extent. However, the urinary aMT6s levels in patients consuming a standard salt diet-but not patients consuming a low salt diet-were significantly and negatively correlated with the daytime and nighttime urinary albumin and protein levels. Contrarily, no significant correlations were found between the urinary aMT6s levels and the BP levels, renal function, and plasma angiotensin II levels in patients consuming either a standard salt or low salt diet. A multiple regression analysis adjusted for age, sex, and body mass index revealed that the urinary albumin and protein levels were significantly and negatively associated with the urinary aMT6s levels in patients consuming a standard salt diet, but not in patients consuming a low salt diet. Conclusion Salt loading aggravates the relationship between melatonin secretion and albuminuria or proteinuria.

Pediatric sleep disturbances and treatment with melatonin

Background

There are no guidelines concerning the best approach to improving sleep, but it has been shown that it can benefit the affected children and their entire families. The aim of this review is to analyse the efficacy and safety of melatonin in treating pediatric insomnia and sleep disturbances.

Main body

Sleep disturbances are highly prevalent in children and, without appropriate treatment, can become chronic and last for many years; however, distinguishing sleep disturbances from normal age-related changes can be a challenge for physicians and may delay treatment. Some published studies have shown that melatonin can be safe and effective not only in the case of primary sleep disorders, but also for sleep disorders associated with various neurological conditions. However, there is still uncertainty concerning dosing regimens and a lack of other data. The dose of melatonin should therefore be individualised on the basis of multiple factors, including the severity and type of sleep problem and the associated neurological pathology.

Conclusions

Melatonin can be safe and effective in treating both primary sleep disorders and the sleep disorders associated with various neurological conditions. However, there is a need for further studies aimed at identifying the sleep disordered infants and children who will benefit most from melatonin treatment, and determining appropriate doses based on the severity and type of disorder.

Effects of Melatonin on the Defense to Acute Hypoxia in Newborn Lambs

Neonatal lambs, as other neonates, have physiologically a very low plasma melatonin concentration throughout 24 h. Previously, we found that melatonin given to neonates daily for 5 days decreased heart weight and changed plasma cortisol and gene expression in the adrenal and heart. Whether these changes could compromise the responses to life challenges is unknown. Therefore, firstly, we studied acute effects of melatonin on the defense mechanisms to acute hypoxia in the neonate. Eleven lambs, 2 weeks old, were instrumented and subjected to an episode of acute isocapnic hypoxia, consisting of four 30 min periods: normoxia (room air), normoxia after an i.v. bolus of melatonin (0.27 mg kg^-1, n = 6) or vehicle (ethanol 1:10 NaCl 0.9%, n = 5), hypoxia (PaO₂: 30 ± 2 mmHg), and recovery (room air). Mean pulmonary and systemic blood pressures, heart rate, and cardiac output were measured, and systemic and pulmonary vascular resistance and stroke volume were calculated. Blood samples were taken every 30 min to measure plasma norepinephrine, cortisol, glucose, triglycerides, and redox markers (8-isoprostane and FRAP). Melatonin blunted the increase of pulmonary vascular resistance triggered by hypoxia, markedly exacerbated the heart rate response, decreased heart stroke volume, and lessened the magnitude of the increase of plasmatic norepinephrine and cortisol levels induced by hypoxia. No changes were observed in pulmonary blood pressure, systemic blood pressures and resistance, cardiac output, glucose, triglyceride plasma concentrations, or redox markers. Melatonin had no effect on cardiovascular, endocrine, or metabolic variables, under normoxia. Secondly, we examined whether acute melatonin administration under normoxia could have an effect in gene expression on the adrenal, lung, and heart. Lambs received a bolus of vehicle or melatonin and were euthanized 30 min later to collect tissues. We found that melatonin affected expression of the immediate early genes egr1 in adrenal, ctgf in lung, and nr3c1, the glucocorticoid receptor, in adrenal and heart. We speculate that these early gene responses may contribute to the observed alterations of the newborn defense mechanisms to hypoxia. This could be particularly important since the use of melatonin is proposed for several diseases in the neonatal period in humans.

Antioxidant Melatonin: Potential Functions in Improving Cerebral Autoregulation After Subarachnoid Hemorrhage

Subarachnoid hemorrhage (SAH) is a subtype of stroke with high mortality and morbidity. Impaired cerebral autoregulation following SAH has been reported owing to effects on sympathetic control, endothelial function, myogenic response, and cerebral metabolism. Impaired cerebral autoregulation is associated with early brain injury, cerebral vasospasm/delayed cerebral ischemia, and SAH prognosis. However, few drugs have been reported to improve cerebral autoregulation after SAH. Melatonin is a powerful antioxidant that is effective (easily crosses the blood brain barrier) and safe (tolerated in large doses without toxicity). Theoretically, melatonin may impact the control mechanisms of cerebral autoregulation via antioxidative effects, protection of endothelial cell integrity, suppression of sympathetic nerve activity, increase in nitric oxide bioavailability, mediation of the myogenic response, and amelioration of hypoxemia. Furthermore, melatonin may have a comprehensive effect on cerebral autoregulation. This review discusses the potential effects of melatonin on cerebral autoregulation following SAH, in terms of the association between pharmacological activities and the mechanisms of cerebral autoregulation.

Deciphering the Function of the Blunt Circadian Rhythm of Melatonin in the Newborn Lamb: Impact on Adrenal and Heart

Neonatal lambs, as with human and other neonates, have low arrhythmic endogenous levels of melatonin for several weeks until they start their own pineal rhythm of melatonin production at approximately 2 weeks of life. During pregnancy, daily rhythmic transfer of maternal melatonin to the fetus has important physiological roles in sheep, nonhuman primates, and rats. This melatonin rhythm provides a circadian signal and also participates in adjusting the physiology of several organs in preparation for extrauterine life. We propose that the ensuing absence of a melatonin rhythm plays a role in neonatal adaptation. To test this hypothesis, we studied the effects of imposing a high-amplitude melatonin rhythm in the newborn lamb on (1) clock time-related changes in cortisol and plasma variables and (2) clock time-related changes of gene expression of clock genes and selected functional genes in the adrenal gland and heart. We treated newborn lambs with a daily oral dose of melatonin (0.25 mg/kg) from birth to 5 days of age, recreating a high-amplitude melatonin rhythm. This treatment suppressed clock time-related changes of plasma adrenocorticotropic hormone, cortisol, clock gene expression, and functional genes in the newborn adrenal gland. In the heart, it decreased heart/body weight ratio, increased expression of Anp and Bnp, and resulted in different heart gene expression from control newborns. The interference of this postnatal melatonin treatment with the normal postnatal pattern of adrenocortical function and heart development support a physiological role for the window of flat postnatal melatonin levels during the neonatal transition.

Melatonin and Human Cardiovascular Disease

The possible therapeutic role of melatonin in the pathophysiology of coronary artery disorder (CAD) is increasingly being recognized. In humans, exogenous melatonin has been shown to decrease nocturnal hypertension, improve systolic and diastolic blood pressure, reduce the pulsatility index in the internal carotid artery, decrease platelet aggregation, and reduce serum catecholamine levels. Low circulating levels of melatonin are reported in individuals with CAD, arterial hypertension, and congestive heart failure. This review assesses current literature on the cardiovascular effects of melatonin in humans. It can be concluded that melatonin deserves to be considered in clinical trials evaluating novel therapeutic interventions for cardiovascular disorders.

High Prevalence of Orthostatic Dysregulation among Circadian Rhythm Disorder Patients

STUDY OBJECTIVES

Patients with circadian rhythm sleep disorders (CRSDs) often have coincidence of orthostatic dysregulation (OD). Both disorders have many common clinical features. However, the prevalence of OD in patients with CRSD has not been examined.

METHODS

Thirty-eight patients with CRSD with either delayed sleep phase disorder or free-running disorder were tested for OD using the new orthostatic test, which was originally established by Tanaka et al. (< 20 years) and the Schellong test, i.e., the active standing test (≥ 20 years).

RESULTS

The overall prevalence of OD in patients with CRSD was 57.9% (22/38), and prevalence of OD was 70% in patients under 20 years of age (14/20). These rates exceed the previously reported values in adolescents aged 14-15 years (15%), regarded as the age with highest OD prevalence. Prevalence was not significantly associated with CRSD severity and medications used.

CONCLUSIONS

We observed a high prevalence of OD in patients with CRSD, suggesting some relationship between CRSD and OD. Large-scale case-control studies are warranted to investigate the underlying mechanisms for this comorbidity.

Potential pleiotropic beneficial effects of adjuvant melatonergic treatment in posttraumatic stress disorder

Loss of circadian rhythmicity fundamentally affects the neuroendocrine, immune, and autonomic system, similar to chronic stress and may play a central role in the development of stress-related disorders. Recent articles have focused on the role of sleep and circadian disruption in the pathophysiology of posttraumatic stress disorder (PTSD), suggesting that chronodisruption plays a causal role in PTSD development. Direct and indirect human and animal PTSD research suggests circadian system-linked neuroendocrine, immune, metabolic and autonomic dysregulation, linking circadian misalignment to PTSD pathophysiology. Recent experimental findings also support a specific role of the fundamental synchronizing pineal hormone melatonin in mechanisms of sleep, cognition and memory, metabolism, pain, neuroimmunomodulation, stress endocrinology and physiology, circadian gene expression, oxidative stress and epigenetics, all processes affected in PTSD. In the current paper, we review available literature underpinning a potentially beneficiary role of an add-on melatonergic treatment in PTSD pathophysiology and PTSD-related symptoms. The literature is presented as a narrative review, providing an overview on the most important and clinically relevant publications. We conclude that adjuvant melatonergic treatment could provide a potentially promising treatment strategy in the management of PTSD and especially PTSD-related syndromes and comorbidities. Rigorous preclinical and clinical studies are needed to validate this hypothesis.

Current role of melatonin in pediatric neurology: clinical recommendations

BACKGROUND/PURPOSE

Melatonin, an indoleamine secreted by the pineal gland, plays a key role in regulating circadian rhythm. It has chronobiotic, antioxidant, anti-inflammatory and free radical scavenging properties.

METHODS

A conference in Rome in 2014 aimed to establish consensus on the roles of melatonin in children and on treatment guidelines.

RESULTS AND CONCLUSION

The best evidence for efficacy is in sleep onset insomnia and delayed sleep phase syndrome. It is most effective when administered 3-5 h before physiological dim light melatonin onset. There is no evidence that extended-release melatonin confers advantage over immediate release. Many children with developmental disorders, such as autism spectrum disorder, attention-deficit/hyperactivity disorder and intellectual disability have sleep disturbance and can benefit from melatonin treatment. Melatonin decreases sleep onset latency and increases total sleep time but does not decrease night awakenings. Decreased CYP 1A2 activity, genetically determined or from concomitant medication, can slow metabolism, with loss of variation in melatonin level and loss of effect. Decreasing the dose can remedy this. Animal work and limited human data suggest that melatonin does not exacerbate seizures and might decrease them. Melatonin has been used successfully in treating headache. Animal work has confirmed a neuroprotective effect of melatonin, suggesting a role in minimising neuronal damage from birth asphyxia; results from human studies are awaited. Melatonin can also be of value in the performance of sleep EEGs and as sedation for brainstem auditory evoked potential assessments. No serious adverse effects of melatonin in humans have been identified.

Melatonin reduces tachycardia in postural tachycardia syndrome: a randomized, crossover trial

BACKGROUND

Postural tachycardia syndrome (POTS) induces disabling chronic orthostatic intolerance with an excessive increase in heart rate (HR) upon standing, and many POTS patients have a hyperadrenergic state. Medications that restrain HR are a promising approach to this problem.

OBJECTIVE

We tested the hypothesis that melatonin will attenuate the tachycardia and improve symptom burden in patients with POTS.

METHODS

Patients with POTS (n = 78) underwent acute drug trials with melatonin 3 mg orally and placebo, on separate mornings, in a randomized crossover design. Blood pressure, HR, and symptoms were assessed while seated and after standing for up to 10 min prior to, and hourly for 4 h following study drug administration.

RESULTS

The reduction in standing HR was significantly greater 2 h after melatonin compared with placebo (P = 0.017). There was no significant difference in the reduction of systolic blood pressure between melatonin and placebo, either with standing or while seated. The symptom burden was not improved with melatonin compared with placebo.

CONCLUSION

Oral melatonin produced a modest decrease in standing tachycardia in POTS. Further research is needed to determine the effects of regular night-time use of this medication in POTS.

Over-the-counter and prescription sleep medication and incident stroke: the REasons for Geographic and Racial Differences in Stroke study

BACKGROUND

Preliminary evidence suggests sleep medications are associated with risk of vascular events; however, the long-term vascular consequences are understudied. This study investigated the relation between sleep medication use and incident stroke.

METHODS

Within the REasons for Geographic And Racial Differences in Stroke study, 21,678 black participants and white participants (≥45 years) with no history of stroke were studied. Participants were recruited from 2003 to 2007. From 2008 to 2010, participants self reported their prescription and over-the-counter sleep medication use over the past month. Suspected stroke events were identified by telephone contact at 6-month intervals and associated medical records were retrieved and physician-adjudicated. Proportional hazards analysis was used to estimate hazard ratios for incident stroke associated with sleep medication use (0, 1-14, and 15+ days per month) controlling for sociodemographics, stroke risk factors, mental health symptoms, and sleep apnea risk.

RESULTS

At the sleep assessment, 9.6% of the sample used prescription sleep medication and 11.1% used over-the-counter sleep aids. Over an average follow-up of 3.3 ± 1.0 years, 297 stroke events occurred. Over-the-counter sleep medication use was associated with increased risk of incident stroke in a frequency-response relationship (P = .014), with a 46% increased risk for 1-14 days of use per month (hazards ratio [HR] = 1.46; 95% confidence interval [CI], .99-2.15) and a 65% increased risk for 15+ days (HR = 1.65; 95% CI, .96-2.85). There was no significant association with prescription sleep medications (P = .80).

CONCLUSIONS

Over-the-counter sleep medication use may independently increase the risk of stroke beyond other risk factors in middle-aged to older individuals with no history of stroke.

Impact of Maternal Melatonin Suppression on Amount and Functionality of Brown Adipose Tissue (BAT) in the Newborn Sheep

In human and sheep newborns, brown adipose tissue (BAT) accrued during fetal development is used for newborn thermogenesis. Here, we explored the role of maternal melatonin during gestation on the amount and functionality of BAT in the neonate. We studied BAT from six lambs gestated by ewes exposed to constant light from 63% gestation until delivery to suppress melatonin (LL), six lambs gestated by ewes exposed to LL but receiving daily oral melatonin (12 mg at 1700 h, LL + Mel) and another six control lambs gestated by ewes maintained in 12 h light:12 h dark (LD). Lambs were instrumented at 2 days of age. At 4-6 days of age, they were exposed to 24°C (thermal neutrality conditions) for 1 h, 4°C for 1 h, and 24°C for 1 h. Afterward, lambs were euthanized and BAT was dissected for mRNA measurement, histology, and ex vivo experiments. LL newborns had lower central BAT and skin temperature under thermal neutrality and at 4°C, and higher plasma norepinephrine concentration than LD newborns. In response to 4°C, they had a pronounced decrease in skin temperature and did not increase plasma glycerol. BAT weight in LL newborns was about half of that of LD newborns. Ex vivo, BAT from LL newborns showed increased basal lipolysis and did not respond to NE. In addition, expression of adipogenic/thermogenic genes (UCP1, ADBR3, PPARγ, PPARα, PGC1α, C/EBPβ, and perilipin) and of the clock genes Bmal1, Clock, and Per2 was increased. Remarkably, the effects observed in LL newborns were absent in LL + Mel newborns. Thus, our results support that maternal melatonin during gestation is important in determining amount and normal functionality of BAT in the neonate.

Is there diurnal variation of the vestibulosympathetic reflex: implications for orthostatic hypotension

Incidences of adverse cardiac events and orthostatic hypotension are associated with diurnal variations. The primary purpose of the present study was to determine if the vestibulosympathetic reflex (VSR) follows a diurnal variation in humans. We hypothesized that the VSR would be attenuated at night based on the relation between melatonin and the VSR. Arterial blood pressure, heart rate, calf blood flow, and muscle sympathetic nerve activity (MSNA) were measured in nine healthy subjects (28 ± 1 yr, 5 men and 4 women) at rest and during head-down rotation. Each subject was tested during the day at 11:34 ± 13 and again at night 22:10 ± 5. MSNA was significantly decreased at night compared with day (8 ± 1 vs. 11 ± 2 bursts/min, respectively, P < 0.02). Heart rate and arterial blood pressure at rest were significantly increased at night compared with day (heart rate: 70 ± 4 vs. 66 ± 4 beats/min and mean arterial blood pressure: 91 ± 2 vs. 87 ± 1 mmHg, respectively). MSNA and hemodynamic responses to head-down rotation were not significantly altered at night compared with day (changes of 3 ± 1 bursts/min and 25 ± 6% for MSNA and calf blood flow, respectively). The data indicate that MSNA at rest decreases during the late evening hours and exhibits a diurnal variation, whereas the VSR does not. In summary, diurnal variation of orthostatic hypotension in humans does not appear to be associated with changes in the VSR and MSNA at rest.

Potency of melatonin in living beings

Living beings are surrounded by various changes exhibiting periodical rhythms in environment. The environmental changes are imprinted in organisms in various pattern. The phenomena are believed to match the external signal with organisms in order to increase their survival rate. The signals are categorized into circadian, seasonal, and annual cycles. Among the cycles, the circadian rhythm is regarded as the most important factor because its periodicity is in harmony with the levels of melatonin secreted from pineal gland. Melatonin is produced by the absence of light and its presence displays darkness. Melatonin plays various roles in creatures. Therefore, this review is to introduce the diverse potential ability of melatonin in manifold aspects in living organism.

Melatonin attenuates the skin sympathetic nerve response to mental stress

Melatonin attenuates muscle sympathetic nerve responses to sympathoexcitatory stimuli, but it is unknown whether melatonin similarly attenuates reflex changes in skin sympathetic nerve activity (SSNA). In this double-blind, placebo-controlled, crossover study, we tested the hypothesis that melatonin (3 mg) would attenuate the SSNA response to mental stress (mental arithmetic). Twelve healthy subjects underwent experimental testing on two separate days. Three minutes of mental stress occurred before and 45 min after ingestion of melatonin (3 mg) or placebo. Skin temperature was maintained at 34°C. Reflex increases in SSNA (peroneal nerve), mean arterial pressure, and heart rate (HR) to mental stress before and after melatonin were determined. Melatonin lowered HR (pre, 66 ± 3 beats/min; and post, 62 ± 3 beats/min, P = 0.046) and SSNA (pre, 14,282 ± 3,706 arbitrary units; and post, 9,571 ± 2,609 arbitrary units, P = 0.034) at rest. In response to mental stress, SSNA increases were significantly attenuated following melatonin ingestion (second minute, 114 ± 30 vs. 74 ± 14%; and third minute, 111 ± 29 vs. 54 ± 12%, both P < 0.05). The mean arterial pressure increase to mental stress was blunted in the third minute (20 ± 2 vs. 17 ± 2 mmHg, P = 0.032), and the HR increase was blunted in the first minute (33 ± 3 vs. 29 ± 3 beats/min, P = 0.034) after melatonin. In summary, exogenous melatonin attenuates the SSNA response to mental stress.

Administration-time differences in effects of hypertension medications on ambulatory blood pressure regulation

Specific features of the 24-h blood pressure (BP) pattern are linked to progressive injury of target tissues and risk of cardiovascular disease (CVD) events. Several studies have consistently shown an association between blunted asleep BP decline and risk of fatal and nonfatal CVD events. Thus, there is growing focus on ways to properly control BP during nighttime sleep as well as during daytime activity. One strategy, termed chronotherapy, entails the timing of hypertension medications to endogenous circadian rhythm determinants of the 24-h BP pattern. Significant and clinically meaningful treatment-time differences in the beneficial and/or adverse effects of at least six different classes of hypertension medications, and their combinations, are now known. Generally, calcium channel blockers (CCBs) are more effective with bedtime than morning dosing, and for dihydropyridine derivatives bedtime dosing significantly reduces risk of peripheral edema. The renin-angiotensin-aldosterone system is highly circadian rhythmic and activates during nighttime sleep. Accordingly, evening/bedtime ingestion of the angiotensin-converting enzyme inhibitors (ACEIs) benazepril, captopril, enalapril, lisinopril, perindopril, quinapril, ramipril, spirapril, trandolapril, and zofenopril exerts more marked effect on the asleep than awake systolic (SBP) and diastolic (DBP) BP means. Likewise, the bedtime, in comparison with morning, ingestion schedule of the angiotensin-II receptor blockers (ARBs irbesartan, olmesartan, telmisartan, and valsartan exerts greater therapeutic effect on asleep BP, plus significant increase in the sleep-time relative BP decline, with the additional benefit, independent of drug terminal half-life, of converting the 24-h BP profile into a more normal dipping pattern. This is the case also for the bedtime versus upon-awakening regimen of combination ARB-CCB, ACEI-CCB, and ARB-diuretic medications. The chronotherapy of conventional hypertension medications constitutes a new and cost-effective strategy for enhancing the control of daytime and nighttime SBP and DBP levels, normalizing the dipping status of their 24-h patterning, and potentially reducing the risk of CVD events and end-organ injury, for example, of the blood vessels and tissues of the heart, brain, kidney, and retina.

Melatonin does not attenuate dynamic cardiovascular and cerebrovascular reflex responses to acute hypotension in healthy men

Background

Melatonin has been shown to attenuate the reflex sympathetic increases that arise in response to orthostatic challenges. We tested the hypothesis that the attenuated sympathetic increase induced by melatonin premedication may weaken the arterial blood pressure (ABP) preserving the capability during acute hypotension, thereby altering dynamic cerebral autoregulation and causing a further decrease in cerebral blood flow (CBF).

Methods

Acute hypotension was induced in 12 healthy subjects by releasing bilateral thigh cuffs before and after an oral dose of melatonin (0.2 mg/kg). Heart rate (HR), arterial blood pressure (ABP), Modelflow estimate of cardiac output (CO), total peripheral resistance (TPR) and cerebral blood flow velocity (CBFV) by transcranial Doppler were measured.

Results

Steady state HR, the mean arterial pressure and CBFV were not altered 60 minutes after melatonin ingestion. Reduced systolic arterial pressure (ΔSAP), changes in HR (ΔHR), CO (ΔCO), and TPR (ΔTPR), ΔHR/ΔSAP and percentage restoration of SAP were not affected after a temporal decrease in ABP induced by thigh cuff release. In the cerebral circulation, melatonin did not affect changes in CBFV, cerebrovascular resistance index, the rate of regulation and percentage restoration of CBFV following a sudden decrease in ABP.

Conclusions

Contrary to our hypothesis, melatonin did not affect the rapid vasodilatory and recovery responses of cardiovascular and dynamic cerebral autoregulation. These results suggest that melatonin premedication may not impair ABP and CBF preserving capability induced by sudden postural changes or hemorrhage.

Effect of melatonin on nocturnal blood pressure: meta-analysis of randomized controlled trials

Background

Patients with nocturnal hypertension are at higher risk for cardiovascular complications such as myocardial infarction and cerebrovascular insult. Published studies inconsistently reported decreases in nocturnal blood pressure with melatonin.

Methods

A meta-analysis of the efficacy and safety of exogenous melatonin in ameliorating nocturnal blood pressure was performed using a random effects model of all studies fitting the inclusion criteria, with subgroup analysis of fast-release versus controlled-release preparations.

Results

Seven trials (three of controlled-release and four of fast-release melatonin) with 221 participants were included. Meta-analysis of all seven studies did not reveal significant effects of melatonin versus placebo on nocturnal blood pressure. However, subgroup analysis revealed that controlled-release melatonin significantly reduced nocturnal blood pressure whereas fast-release melatonin had no effect. Systolic blood pressure decreased significantly with controlled-release melatonin (−6.1 mmHg; 95% confidence interval [CI] −10.7 to −1.5; P = 0.009) but not fast-release melatonin (−0.3 mmHg; 95% CI −5.9 to 5.30; P = 0.92). Diastolic blood pressure also decreased significantly with controlled-release melatonin (−3.5 mmHg; 95% CI −6.1 to −0.9; P = 0.009) but not fast-release melatonin (−0.2 mmHg; 95% CI −3.8 to 3.3; P = 0.89). No safety concerns were raised.

Conclusion

Add-on controlled-release melatonin to antihypertensive therapy is effective and safe in ameliorating nocturnal hypertension, whereas fast-release melatonin is ineffective. It is necessary that larger trials of longer duration be conducted in order to determine the long-term beneficial effects of controlled-release melatonin in patients with nocturnal hypertension.

Intracoronary melatonin increases coronary blood flow and cardiac function through β-adrenoreceptors, MT1/MT2 receptors, and nitric oxide in anesthetized pigs

Melatonin is involved in the regulation of the cardiovascular system through the modulation of sympathetic function and the nitric oxide (NO)-related pathway and interaction with MT1/MT2 receptors. However, information regarding its direct actions on coronary blood flow and cardiac function is scarce. This study therefore determined the primary in vivo effect of melatonin on cardiac function and perfusion and the involvement of the autonomic nervous system, MT1/MT2 receptors, and NO. In 35 pigs, melatonin infused into the coronary artery at 70 pg for each mL/min of coronary blood flow while preventing changes in heart rate and arterial pressure increased coronary blood flow, dP/dt(max), segmental shortening, and cardiac output by about 12%, 14%, 8%, and 23% of control values (P < 0.05), respectively. These effects were accompanied by an increase in coronary NO release of about 46% (P < 0.05) of control values. The aforementioned responses were graded in a further five pigs. Moreover, the blockade of muscarinic cholinoreceptors (n = 5) and α-adrenoreceptors (n = 5) did not abolish the observed responses to melatonin. After β(1)-adrenoreceptors blocking (n = 5), melatonin failed to affect cardiac function, whereas β(2)-adrenoreceptors (n = 5) and NO synthase inhibition (n = 5) prevented the coronary response and the effect of melatonin on NO release. Finally, all effects were prevented by MT1/MT2 receptor inhibitors (n = 10). In conclusion, melatonin primarily increased coronary blood flow and cardiac function through the involvement of MT1/MT2 receptors, β-adrenoreceptors, and NO release. These findings add new information about the mechanisms through which melatonin physiologically modulates cardiovascular function and exerts cardioprotective effects.

Melatonin differentially affects vascular blood flow in humans

Melatonin is synthesized and released into the circulation by the pineal gland in a circadian rhythm. Melatonin has been demonstrated to differentially alter blood flow to assorted vascular beds by the activation of different melatonin receptors in animal models. The purpose of the present study was to determine the effect of melatonin on blood flow to various vascular beds in humans. Renal (Doppler ultrasound), forearm (venous occlusion plethysmography), and cerebral blood flow (transcranial Doppler), arterial blood pressure, and heart rate were measured in 10 healthy subjects (29±1 yr; 5 men and 5 women) in the supine position for 3 min. The protocol began 45 min after the ingestion of either melatonin (3 mg) or placebo (sucrose). Subjects returned at least 2 days later at the same time of day to repeat the trial after ingesting the other substance. Melatonin did not alter heart rate and mean arterial pressure. Renal blood flow velocity (RBFV) and renal vascular conductance (RVC) were lower during the melatonin trial compared with placebo (RBFV, 40.5±2.9 vs. 45.4±1.5 cm/s; and RVC, 0.47±0.02 vs. 0.54±0.01 cm·s(-1)·mmHg(-1), respectively). In contrast, forearm blood flow (FBF) and forearm vascular conductance (FVC) were greater with melatonin compared with placebo (FBF, 2.4±0.2 vs. 1.9±0.1 ml·100 ml(-1)·min(-1); and FVC, 0.029±0.003 vs. 0.023±0.002 arbitrary units, respectively). Melatonin did not alter cerebral blood flow measurements compared with placebo. Additionally, phentolamine (5-mg bolus) after melatonin reversed the decrease in RVC, suggesting that melatonin increases sympathetic outflow to the kidney to mediate renal vasoconstriction. In summary, exogenous melatonin differentially alters vascular blood flow in humans. These data suggest the complex nature of melatonin on the vasculature in humans.

Melatonin attenuates the vestibulosympathetic but not vestibulocollic reflexes in humans: selective impairment of the utricles

Melatonin has been reported to decrease nerve activity of medial vestibular nuclei in the rat and is associated with attenuated muscle sympathetic nerve activity (MSNA) responses to baroreceptor unloading in humans. The purpose of this study was to determine if melatonin alters the vestibulosympathetic reflex (VSR) and vestibulocollic reflex (VCR) in humans. In study 1, MSNA, arterial blood pressure, and heart rate were measured in 12 healthy subjects (28 ± 1 yr; 6 men, 6 women) during head-down rotation (HDR) before and 45 min after ingestion of either melatonin (3 mg) or placebo (sucrose). Subjects returned at least 2 days later at the same time of day to repeat the trial after ingesting the opposite treatment (melatonin or placebo). Melatonin significantly attenuated MSNA responses during HDR compared with placebo (burst frequency Δ 4 ± 1 vs. Δ 7 ± 1 bursts/min, and total MSNA Δ 51 ± 20 and Δ 96 ± 15%, respectively; P < 0.02). In study 2, vestibular evoked myogenic potentials (VEMP) were measured in 10 healthy subjects (26 ± 1 yr; 4 men and 6 women) before and after ingestion of 3 mg melatonin. Melatonin did not alter the timing of the p13 and n23 peaks (pre-melatonin 13.2 ± 0.4 and 21.3 ± 0.6 ms vs. post-melatonin 13.5 ± 0.4 and 21.4 ± 0.7 ms, respectively) or the p13-n23 interpeak amplitudes [pre-melatonin 22.5 ± 4.6 arbitrary units (au) and post-melatonin 22.7 ± 4.6 au]. In summary, melatonin attenuates the VSR and supports the concept that melatonin negatively affects orthostatic tolerance. However, melatonin does not alter the VCR in humans suggesting melatonin's effect on the VSR appears to be mediated by the utricles.

Photoperiod alters autonomic regulation of the heart

Outside of the tropics, environmental conditions fluctuate in a generally predictable manner across the year. Many small mammals have evolved mechanisms, such as seasonal breeding and annual adjustments in physiology, morphology, and behavior, that promote winter survival when food is scarce and thermoregulation is challenging. Photoperiod (day length) is a cue used by many seasonal breeders to predict seasonal changes in environmental conditions. One system that is uniquely situated to mediate photoperiod-induced alterations in physiology is the autonomic nervous system (ANS). The 2 branches of the ANS are key regulators of immune responses, thermoregulation, and energy balance, functions that undergo marked shifts in baseline and reactivity following acclimation to short day lengths. Although previous studies have investigated the effects of photoperiod on ANS endpoints, this study examined the direct effects of photoperiod on integrated ANS function. To test the hypothesis that short day lengths increase parasympathetic and sympathetic tones, we maintained adult male Siberian hamsters (Phodopus sungorus) to either long or short photoperiods and then analyzed electrocardiogram recordings. Short day lengths increased both parasympathetic tone, as measured by respiratory sinus arrhythmia, and sympathetic control of the heart, measured with autonomic blockade. Additionally, short day lengths enhanced the withdrawal of parasympathetic control and the increase of sympathetic tone in response to acute restraint stress. Finally, these effects were discovered to be independent of circulating androgens. These data indicate that the ANS of Siberian hamsters undergoes profound changes following prolonged exposure to short winter-like day lengths.

Chronobiological characterization of women with primary vasospastic syndrome: body heat loss capacity in relation to sleep initiation and phase of entrainment

Women with primary vasospastic syndrome (VS), but otherwise healthy, exhibit a functional disorder of vascular regulation (main symptom: cold extremities) and often suffer from difficulties initiating sleep (DIS). Diverse studies have shown a close association between distal vasodilatation before lights off and a rapid onset of sleep. Therefore, we hypothesized that DIS in women with VS could be due to a reduced heat loss capacity in the evening, i.e., subjects are physiologically not ready for sleep. The aim of the study was to elucidate whether women having both VS and DIS (WVD) or not (controls) show different circadian characteristics (e.g., phase delay of the circadian thermoregulatory system with respect to the sleep-wake cycle). Healthy young women (n = 9 WVD and n = 9 control) completed a 40-h constant routine protocol (adjusted to habitual bedtime) before and after an 8-h sleep episode. Skin temperatures [off-line calculated as distal-proximal skin temperature gradient (DPG)] and core body temperature (CBT; rectal) were continuously recorded. Half-hourly saliva samples were collected for melatonin assay and subjective sleepiness was assessed on the Karolinska Sleepiness Scale (KSS). Compared with control, WVD showed no differences in habitual bed times, but a 1-h circadian phase delay of dim light-melatonin onset (hours after lights on: WVD 14.6 +/- 0.3 h; control 13.5 +/- 0.2 h; P = 0.01). Similar phase shifts were observed in CBT, DPG, and KSS ratings. In conclusion, WVD exhibit a phase delay of the endogenous circadian system with respect to their habitual sleep-wake cycle, which could be a cause of DIS.

Thermoregulatory effects of melatonin in relation to sleepiness

Thermoregulatory processes have long been implicated in the initiation of human sleep. In this paper, we review our own studies conducted over the last decade showing a crucial role for melatonin as a mediator between the thermoregulatory and arousal system in humans. Distal heat loss, via increased skin temperature, seems to be intimately coupled with increased sleepiness and sleep induction. Exogenous melatonin administration during the day when melatonin is essentially absent mimics the endogenous thermophysiological processes occurring in the evening and induces sleepiness. Using a cold thermic challenge test, it was shown that melatonin-induced sleepiness occurs in parallel with reduction in the thermoregulatory set-point (threshold); thus, melatonin may act as a circadian modulator of the thermoregulatory set-point. In addition, an orthostatic challenge can partially block the melatonin-induced effects, suggesting an important role of the sympathetic nervous system as a link between the thermoregulatory and arousal systems. A topographical analysis of finger skin temperature with infrared thermometry revealed that the most distal parts of the fingers, i.e., fingertips, represent the important skin regions for heat loss regulation, most probably via opening the arteriovenous anastomoses, and this is clearly potentiated by melatonin. Taken together, melatonin is involved in the fine-tuning of vascular tone in selective vascular beds, as circulating melatonin levels rise and fall throughout the night. Besides the role of melatonin as "nature's soporific", it can also serve as nature's nocturnal vascular modulator.

Melatonin does not inhibit hypothalamic-pituitary-adrenal activity in waking young men

The pineal hormone melatonin is mainly secreted during night-time which, in humans, is the normal time of sleep. It has been proposed that, during this period, melatonin exerts an inhibitory influence on secretory activity of the hypothalamic-pituitary-adrenal (HPA) system, although there is little evidence for this view in humans. In blind humans, a single oral dose of melatonin at bed time suppressed nocturnal cortisol secretion. However, suppression could have been secondary to an improved sleep after melatonin in these experiments. In the present study, we examined whether melatonin exerts a similar inhibitory effect on HPA activity in waking subjects. Fourteen healthy young men were tested at bed time, but kept awake throughout the experimental epoch. Thirty minutes after oral ingestion of 5 mg melatonin, activity of the HPA-system was stimulated through a standard insulin-induced hypoglycaemia. Adrenocorticotrophin hormone and cortisol concentrations under basal conditions before insulin injection, as well as in response to insulin-induced hypoglycaemia, were almost identical for the melatonin and placebo control conditions (P > 0.5). However, melatonin increased plasma prolactin concentrations (P < 0.01) and reduced systolic blood pressure in the time interval following hypoglycaemia (P < 0.05). Based on a review of the literature and our results, we conclude that melatonin per se has no substantially suppressing effect on HPA secretory activity, although such an effect can be gated by sleep-related processes.

Chronic melatonin therapy fails to alter amyloid burden or oxidative damage in old Tg2576 mice: implications for clinical trials

Melatonin has been proposed as a treatment for Alzheimer's disease based on the demonstration of antioxidant and "anti-amyloid" effects in vitro and in vivo. Chronic melatonin therapy in old, amyloid plaque-bearing transgenic mice was studied. Tg2576 mice started melatonin treatment at 14 months of age. After 4 months of treatment, there were no differences between untreated and melatonin-treated mice in cortical levels of soluble, formic acid extracted, or histologically detectable beta amyloid (Abeta), nor in brain levels of lipid peroxidation product (total 8,12-isoprostane F(2alpha)-VI), despite marked elevations in plasma melatonin. We conclude that melatonin fails to produce anti-amyloid or antioxidant effects when initiated after the age of amyloid plaque deposition. These findings diminish the possibility that melatonin will be useful for the treatment of established Alzheimer's disease.