Heart Rate Orcadian Rhythm as a Biological Marker of Desynchronization in Major Depression: A Methodological and Preliminary Report

The blues and rhythm

Most organisms, including humans, show daily rhythms in many aspects of physiology and behavior, and abnormalities in the rhythms are potential risk factors for various diseases. Mood disorders such as depression are no exception. Accumulating evidence suggests strong associations between circadian disturbances and the development of depression. Numerous studies have shown that interventions to circadian rhythms trigger depression-like phenotypes in human cases and animal models. Conversely, mood changes can affect circadian rhythms as symptoms of depression. Our preliminary data suggest that the phosphorylation signal pathway of the clock protein may act as a common pathway for mood and clock regulation. We hypothesize that mood regulation and circadian rhythms may influence each other and may share a common regulatory mechanism. This review provides an overview of circadian disturbances in animal models and human patients with depression.

Molecular Link between Circadian Rhythmicity and Mood Disorders

BACKGROUND

The internal clock is driven by circadian genes [e.g., Clock, Bmal1, Per1-3, Cry1-2], hormones [e.g., melatonin, cortisol], as well as zeitgeber ['synchronisers']. Chronic disturbances in the circadian rhythm in patients diagnosed with mood disorders have been recognised for more than 50 years.

OBJECTIVES

The aim of this review is to summarise the current knowledge and literature regarding circadian rhythms in the context of mood disorders, focussing on the role of circadian genes, hormones, and neurotransmitters.

METHOD

The review presents the current knowledge and literature regarding circadian rhythms in mood disorders using the Pubmed database. Articles with a focus on circadian rhythms and mood disorders [n=123], particularly from 1973 to 2020, were included.

RESULTS

The article suggests a molecular link between disruptions in the circadian rhythm and mood disorders. Circadian disturbances, caused by the dysregulation of circadian genes, hormones, and neurotransmitters, often result in a clinical picture resembling depression.

CONCLUSION

Circadian rhythms are intrinsically linked to affective disorders, such as unipolar depression and bipolar disorder.

Complexity Variability Assessment of Nonlinear Time-Varying Cardiovascular Control

The application of complex systems theory to physiology and medicine has provided meaningful information about the nonlinear aspects underlying the dynamics of a wide range of biological processes and their disease-related aberrations. However, no studies have investigated whether meaningful information can be extracted by quantifying second-order moments of time-varying cardiovascular complexity. To this extent, we introduce a novel mathematical framework termed complexity variability, in which the variance of instantaneous Lyapunov spectra estimated over time serves as a reference quantifier. We apply the proposed methodology to four exemplary studies involving disorders which stem from cardiology, neurology and psychiatry: Congestive Heart Failure (CHF), Major Depression Disorder (MDD), Parkinson's Disease (PD), and Post-Traumatic Stress Disorder (PTSD) patients with insomnia under a yoga training regime. We show that complexity assessments derived from simple time-averaging are not able to discern pathology-related changes in autonomic control, and we demonstrate that between-group differences in measures of complexity variability are consistent across pathologies. Pathological states such as CHF, MDD, and PD are associated with an increased complexity variability when compared to healthy controls, whereas wellbeing derived from yoga in PTSD is associated with lower time-variance of complexity.

Nighttime heart rate predicts response to depression treatment in patients with coronary heart disease

BACKGROUND

Previous studies suggest that patients with coronary heart disease (CHD) who do not respond to treatment for depression are at higher risk of mortality than are treatment responders. The purpose of this study was to determine whether elevated nighttime heart rate (HR) and low heart rate variability (HRV), both of which have been associated with depression and with cardiac events in patients with CHD, predict poor response to depression treatment in patients with CHD.

METHODS

Patients with stable CHD and a current major depressive episode completed 24h ambulatory ECG monitoring and were then treated for up to 16 weeks with cognitive behavior therapy (CBT), either alone or in combination with an antidepressant. Pre-treatment HR and HRV were calculated for 124 patients who had continuous ECG from early evening to mid-morning.

RESULTS

Following treatment, 64 of the 124 patients (52%) met study criteria for remission (Hamilton Rating Scale for Depression score≤7). Prior to treatment, non-remitters had higher nighttime HR (p=0.03) and lower nighttime HRV (p=0.01) than did the remitters, even after adjusting for potential confounds.

LIMITATIONS

Polysomnography would have provided information about objective sleep characteristics and sleep disorders. More CBT sessions and higher doses of antidepressants may have resulted in more participants in remission.

CONCLUSIONS

High nighttime HR and low nighttime HRV predict a poor response to treatment of major depression in patients with stable CHD. These findings may help explain why patients with CHD who do not respond to treatment are at higher risk for mortality.

"Chrono-functional milk": The difference between melatonin concentrations in night-milk versus day-milk under different night illumination conditions

Pineal melatonin (MLT) is produced at highest levels during the night, under dark conditions. We evaluated differences in MLT-concentration by comparing daytime versus night time milk samples, from two dairy farms with different night illumination conditions: (1) natural dark (Dark-Night); (2) short wavelength Artificial Light at Night (ALAN, Night-Illuminated). Samples were collected from 14 Israeli Holstein cows from each commercial dairy farm at 04:30 h ("Night-milk") 12:30 h ("Day-milk") and analyzed for MLT-concentration. In order to study the effects of night illumination conditions on cows circadian rhythms, Heart Rate (HR) daily rhythms were recorded. MLT-concentrations of Night-milk samples from the dark-night group were significantly (p < 0.001) higher than those of Night-illuminated conditions (30.70 ± 1.79 and 17.81 ± 0.33 pg/ml, respectively). Interestingly, night illumination conditions also affected melatonin concentrations at daytime where under Dark-Night conditions values are significantly (p < 0.001) higher than Night-Illuminated conditions, (5.36 ± 0.33 and 3.30 ± 0.18 pg/ml, respectively). There were no significant differences between the two treatments in the milk yield and milk composition except somatic cell count (SCC), which was significantly lower (p = 0.02) in the Dark-Night group compared with the Night-Illuminated group. Cows in both groups presented a significant (p < 0.01) HR daily rhythm, therefore we assume that in the night illuminated cows feeding and milking time are the "time keeper", while in the Dark-night cows, HR rhythms were entrained by the light/dark cycle. The higher MLT-concentration in Dark-night cows with the lower SCC values calls upon farmers to avoid exposure of cows to ALAN. Therefore, under Dark-night conditions milk quality will improve by lowering SCC values where separation between night and day of such milk can produce chrono-functional milk, naturally rich with MLT.

A twin-study of genetic contributions to morningness-eveningness and depression

Circadian rhythms are associated with the preference for sleep-wake timing, also known as morningness-eveningness (ME). Both circadian rhythms and ME are influenced by genetic factors. Studies show an association between eveningness and depression. This study investigates the heritability of ME and whether ME and depression share common genetic influences. Study participants (n = 1237) were from the Vietnam Era Twin Study of Aging, a longitudinal study of aging with a baseline in midlife. Participants received the Morningness-Eveningness Questionnaire (MEQ) and the Center for Epidemiologic Studies Depression (CES-D) Scale as part of an extensive neurocognitive and psychosocial assessment. MEQ correlations between members of twin pairs were 0.41 (95% CI 0.31-0.49) for monozygotic (MZ) twins and 0.28 for dizygotic (DZ) twins (95% CI 0.19-0.41). CES-D correlations were 0.38 (95% CI 0.28-0.46) for MZ twins and 0.24 (95% CI 0.14-0.36) for DZ twins. Greater eveningness (i.e. lower MEQ scores) was significantly related to more depression symptoms (phenotypic correlation = -0.15 (95% CI -0.21 to -0.09). In the best fitting model, the heritability estimates are 0.42 for the MEQ and 0.37 for the CES-D. A significant genetic correlation of -0.21 indicated that ME and depression share a significant amount of their underlying genetic variance. The genetic covariance between ME and depression accounted for 59.1% of the phenotypic correlation. Of the CES-D sub-scales, Depressed Mood and Interpersonal Difficulties were significantly heritable, while only Well-Being had a significant genetic correlation with ME. ME and depression are both heritable (ME 0.42, depression 0.37) and share common genetic factors, suggesting an overlap in etiology and the relevance of circadian rhythms to depression. Further study of this relationship may help elucidate etiological factors in depression and targets for treatment.

Manipulating the sleep-wake cycle and circadian rhythms to improve clinical management of major depression

BACKGROUND

Clinical psychiatry has always been limited by the lack of objective tests to substantiate diagnoses and a lack of specific treatments that target underlying pathophysiology. One area in which these twin failures has been most frustrating is major depression. Due to very considerable progress in the basic and clinical neurosciences of sleep-wake cycles and underlying circadian systems this situation is now rapidly changing.

DISCUSSION

The development of specific behavioral or pharmacological strategies that target these basic regulatory systems is driving renewed clinical interest. Here, we explore the extent to which objective tests of sleep-wake cycles and circadian function - namely, those that measure timing or synchrony of circadian-dependent physiology as well as daytime activity and nighttime sleep patterns - can be used to identify a sub-class of patients with major depression who have disturbed circadian profiles.

SUMMARY

Once this unique pathophysiology is characterized, a highly personalized treatment plan can be proposed and monitored. New treatments will now be designed and old treatments re-evaluated on the basis of their effects on objective measures of sleep-wake cycles, circadian rhythms and related metabolic systems.

Effect of Pedal Rate on Diurnal Variations in Cardiorespiratory Variables

Recently, it was observed that the freely chosen pedal rate of elite cyclists was significantly lower at 06:00 than at 18:00 h, and that ankle kinematics during cycling exhibits diurnal variation. The modification of the pedaling technique and pedal rate observed throughout the day could be brought about to limit the effect of diurnal variation on physiological variables. Imposing a pedal rate should limit the subject's possibility of adaptation and clarify the influence of time of day on physiological variables. The purpose of this study was to determine whether diurnal variation in cardiorespiratory variables depends on pedal rate. Ten male cyclists performed a submaximal 15 min exercise on a cycle ergometer (50% Wmax). Five test sessions were performed at 06:00, 10:00, 14:00, 18:00, and 22:00 h. The exercise bout was divided into three equivalent 5 min periods during which different pedal rates were imposed (70 rev x min(-1), 90 rev x min(-1) and 120 rev x min(-1)). No significant diurnal variation was observed in heart rate and oxygen consumption, whatever the pedal rate. A significant diurnal variation was observed in minute ventilation (p=0.01). In addition, the amplitude of the diurnal variation in minute ventilation depended on pedal rate: the higher the pedal rate, the greater the amplitude of its diurnal variation (p=0.03). The increase of minute ventilation throughout the day is mainly due to variation in breath frequency (p=0.01)--the diurnal variation of tidal volume (all pedal rate conditions taken together) being non-significant--but the effect of pedal rate x time of day interaction on minute ventilation specific to the higher pedal rate conditions (p=0.03) can only be explained by the increase of tidal volume throughout the day. Even though an influence of pedal rate on diurnal rhythms in overall physiological variables was not also evidenced, high pedal rate should have been imposed when diurnal variations of physiological variables in cycling were studied.

Diurnal Variations in Cycling Kinematics

Physiological and biomechanical constraints as well as their fluctuations throughout the day must be considered when studying determinant factors in the preferred pedaling rate of elite cyclists. The aim of this study was to monitor the diurnal variation of spontaneous pedaling rate and movement kinematics over the crank cycle. Twelve male competitive cyclists performed a submaximal exercise on a cycle ergometer for 15 min at 50% of their W(max). Two test sessions were performed at 06:00 and 18:00 h on two separate days to assess diurnal variation in the study variables. For each test session, the exercise bout was divided into three equivalent 5-min periods during which subjects were requested to use different pedal rates (spontaneous cadence, 70 and 90 rev min(-1)). Pedal rate and kinematics data (instantaneous pedal velocity and angle of the ankle) were collected. The results show a higher spontaneous pedal rate in the late afternoon than in the early morning (p < 0.001). For a given pedal rate condition, there was a less variation in pedal velocity during a crank cycle in the morning than in the late afternoon. Moreover, diurnal variations were observed in ankle mobility across the crank cycle, the mean plantar flexion observed throughout the crank cycle being greater in the 18:00 h test session (p < 0.001). These results suggest that muscular activation patterns during a cyclical movement could be under the influence of circadian fluctuations.

Autonomic neuropathy in hemodialysis patients: questionnaires versus clinical tests

AIM

Uremic neuropathy (UN) is a typical complication of long-term dialysis therapy and may manifest as distal, symmetric, and both senso-motoric and autonomic nerve dysfunction. Its clinical picture is highly variable and it is not known how the symptoms of UN reported by chronic dialysis patients correspond to positive results of clinical tests for autonomic dysfunction. The objective of this study was to compare the prevalence of self-reported symptoms of autonomic neuropathy (AN) revealed in patient questionnaires with the prevalence of findings from objective clinical tests.

METHODS

The study group included 45 patients (26 males, 19 females), mean age 60 +/- 15.5 years, on chronic hemodialysis for 7.8 +/- 2.6 years. In all subjects, blood pressure (BPci) and heart rate (HRci) variability indexes (calculated as BP and HR during Ewing and Clarke' tests divided by HR and BP at rest) were measured using the Finapres which records beat-to-beat blood pressure and heart rate. Additionally, patients completed seven-question questionnaires which assessed the presence of most typical AN symptoms. The control group comprised 12 healthy subjects (age 52 +/- 19.5 years).

RESULTS

The results from the questionnaires showed that falls of BP occurred in 73% of patients, itching of skin in 42%, constipation or diarrhea in 33%, filling of stomach in 25%, and decreased sweating in 17%. Altogether 85.9% of patients reported one or more subjective symptoms of neuropathy. In contrast, modified Ewing and Clarke's test revealed that only 17.7% of participants demonstrated overt pathologies. Neither blood pressure nor heart rate changeability indexes in patients and healthy subjects showed significant differences with the exception of intense inhalation and static effort.

CONCLUSIONS

Our results show much higher frequency of subjective symptoms of AN in chronic dialysis patients than objective symptoms detected with clinical tests.

Nighttime heart rate and survival in depressed patients post acute myocardial infarction

OBJECTIVES

To determine if: 1) depressed patients with a recent acute myocardial infarction (AMI) have higher nighttime heart rate (HR) than nondepressed patients, and 2) elevated nighttime HR is associated with decreased survival post AMI. Depression is a risk factor for mortality post AMI. It is also associated with sleep disturbances and with elevated HR, which may be more pronounced at night. Resting and 24-hour HR have been found to predict mortality in patient and community samples.

METHODS

Ambulatory electrocardiographic data were obtained from 333 depressed patients and 383 nondepressed patients with recent AMI. They were followed for up to 30 months (median = 24 months).

RESULTS

Depressed patients had higher nighttime HR (70.7 +/- 0.7 versus 67.7 +/- 0.6 beats per minute (bpm); p = .001), and daytime HR (76.4 +/- 0.7 versus 74.2 +/- 0.6 bpm; p = .02) than nondepressed patients, even after adjusting for potential confounds. Depression (hazard ratio (Haz R) = 2.19; p = .02) and nighttime HR (Haz R = 1.03; p = .004), but not daytime HR, predicted survival after adjusting for other major predictors and for each other. The interaction between nighttime HR and depression on survival approached, but did not achieve, significance (p = .08).

CONCLUSIONS

Mean day and nighttime HR values are higher in depressed patients than in nondepressed patients post AMI. Depression and elevated nighttime HR, but not daytime HR, are independent predictors of survival in these patients. Although depressed patients have a higher nighttime HR than nondepressed patients, nighttime HR predicts mortality in both depressed and nondepressed patients.

Major depressive disorder, sleep EEG and agomelatine: an open-label study

This open study evaluates the effect of agomelatine, a melatonergic receptor agonist and 5-HT2C antagonist antidepressant, on sleep architecture in patients suffering from major depressive disorder. Fifteen outpatients with a baseline HAMD score > or = 20 were treated with 25 mg/d agomelatine for 42 d. Polysomographic studies were performed at baseline, day 7, day 14, and day 42. Sleep efficiency, time awake after sleep onset and the total amount of slow-wave sleep (SWS) increased at week 6. The increase of SWS was predominant during the first sleep cycle. The amount of SWS decreased throughout the first four sleep cycles from day 7 and delta ratio increased from day 14 onwards. No change in rapid eye movement (REM) latency, amount of REM or REM density was observed and agomelatine was well tolerated. In conclusion agomelatine improved sleep continuity and quality. It normalized the distribution of SWS sleep and delta power throughout the night.

A new potential marker for abnormal cardiac physiology in depression

The purpose of the study was to determine if a nonlinear estimate of dimensional complexity, Pointwise correlation dimension (Pd2), could be used to identify abnormal cardiac physiology associated with depression in primary care outpatients. The subjects were 22 medical controls and 30 general medical outpatients with depression who wore a cardiac monitor for 24 h. There was a significant difference between depressed and control subjects for Pd2 based on the entire cardiac time-series, with depressed subjects exhibiting higher Pd2 values. A cutoff score reflecting high dimensional complexity was selected, and an odds ratio was calculated demonstrating that patients with Pd2 values above the cut-off were 8.8 times more likely to fall in the depressed group than the control group. Additional research is needed to determine if Pd2 is useful for identifying physiological markers of depression.

Circadian rhythms during cycling exercise and finger-tapping task

The aim of this study was to follow the circadian fluctuation of the spontaneous pedal rate and the motor spontaneous tempo (MST) in a sample of highly trained cyclists. Ten subjects performed five test sessions at various times of day. During each test session, subjects were required to perform (i) a finger-tapping task, in order to set the MST and (ii) a submaximal exercise on a cycle ergometer for 15 min at 50% of their Wmax. For this exercise, pedal rate was freely chosen. Spontaneous pedal rate and heart rate (HR) were measured continuously. The results demonstrated a circadian variation for mean oral temperature, HR, and MST. Under submaximal exercise conditions, HR showed no significant time-of-day influence although spontaneous pedal rate changed significantly throughout the day. Circadian rhythm of oral temperature and pedal rate were strongly correlated. Moreover, a significant positive correlation was found between MST and pedal rate. Both parameters may be controlled by a common brain oscillator. MST, rest HR, and pedal rate changes follow the rhythm of internal temperature, which is considered to be the major marker in chronobiology, therefore, if there is a relation between MST and pedal rate, we cannot rule out partial dependence of both parameters on body temperature.

Mouse model for morningness/eveningness

Human morning/evening preferences has recently been reported to be associated with polymorphism of the 3' flanking region of the Clock gene, which was the first identified mammalian circadian clock gene. We recorded body temperature, spontaneous activity, electroencephalogram and electromyogram for 48 h in mice with Jcl:ICR genetic background and homozygous for the Clock mutation (Cl/Cl on Jcl:ICR). In both wild-type and Cl/Cl on Jcl:ICR, body temperature, activity, wake and sleep were completely entrained to LD cycle. However, phases of the rhythm for body temperature, activity and wake duration in the Cl/Cl on Jcl:ICR were about 2 h delayed in comparison with the wild-type. This study has provided further evidence on the close relationship between human morning/evening preference and the molecular basis of circadian clock system, and has suggested that Cl/Cl on Jcl:ICR is useful for an animal model for human morning/evening preference.

Is self-reported morbidity related to the circadian clock?

Morningness and eveningness preference, an endogenous component of the circadian clock, is characterized by an interindividual difference in circadian phase and requires of humans a specific timing of behavior. The biological rhythms of morning and evening types are consequently phase shifted with fixed socioeconomic constraints. The impact of this phase shift on health is widely debated. The purpose of the authors' study was to determine the influence of morningness/eveningness preference on self-reported morbidity and health in an active population. A total of 1165 nonshift workers of the French national electrical and gas company, enrolled in the GAZEL cohort and aged 51.3+/-3.3 years, were included in this study. They replied by mail with a completed questionnaire, including morningness/eveningness preference, self-reported morbidity, subjective sleep patterns, and daytime somnolence and sleeping schedules for 3 weeks, during the spring of 1997. Annual self-reported health impairments were assessed with the annual general questionnaire of the GAZEL cohort for 1997. After adjustment for age, sex, and occupational status, morningness-like and eveningness-like participants reported a specific worse self-reported morbidity. Whereas morningness was associated with worse sleep (p = 0.0001), eveningness was associated with feeling less energetic (p = 0.04) and physical mobility (p = 0.02). These relationships were observed even in good sleepers, except for physical mobility. After adjustment for confounding variables, eveningness-like participants reported more sleep (p = 0.0004) and mood (p = 0.00018) disorders than morningness-like participants. Morningness/eveningness preference was related to specific chronic complaints of insomnia: morningness was related with difficulty in maintaining sleep (p = 0.0005) and the impossibility to return to sleep in the early morning (p = 0.0001) (sleep phase-advance syndrome); eveningness was related to difficulty in initiating sleep (p = 0.0001) and morning sleepiness (p = 0.0001). In good sleepers, morningness was related with sleep phase-advance syndrome (p = 0.0001) and eveningness with morning sleepiness (p = 0.0001). In conclusion, the expression (phase advance or delay) of the circadian clock could be related to worse self-reported morbidity and health. These findings must be verified by further epidemiological studies, but they suggest that the impossibility to return to sleep in the early morning is not only associated with age.

Circadian rhythmicity of cortisol and body temperature: morningness-eveningness effects

The purpose of this study was to describe and compare the circadian rhythm of body temperature and cortisol, as well as self-reported clock times of sleep onset and offset on weekdays and weekends in 19 healthy adult "larks" (morning chronotypes) and "owls" (evening chronotypes), defined by the Home and Ostberg questionnaire. Day-active subjects entered the General Clinical Research Center, where blood was sampled every 2 h over 38 h for later analysis for cortisol concentration by enzyme immunoassay. Rectal body temperature was measured continuously. Lights were turned off at 22:30 for sleep and turned on at 06:00, when subjects were awakened. The acrophases (peak times) of the cortisol and temperature rhythms occurred 55 minutes (P < or = .05) and 68 minutes (P < .01), respectively, earlier in the morningness group. The amplitude of the cortisol rhythm was lower in the eveningness than in the morningness group (P = n.s.). Subject groups differed on all indices of habitual and preferred timing of sleep and work weekdays and weekends (P = .05-.001).

Valpromide increases amplitude of heart rate circadian rhythm in remitted bipolar and unipolar disorders. A placebo-controlled study

The aim of this study was to investigate for the effects of valpromide on heart rate circadian rhythm in remitted recurrent unipolar and bipolar disorders (DSM-III-R). It consisted of a comparative, randomized, double-blind, repeated cross-over study of valpromide versus placebo over four four-week periods. The primary evaluation criteria was heart rate (HR). Secondary criteria comprised motor activity (MA) and the Bech and Rafaelsen mania assessment scale, Horne and Ostberg questionnaire, Montgomery and Asberg depression rating scale, Spiegel questionnaire, a sleep diary, and Clinical Global Impression. Fifteen patients were included, giving 60 one-month periods (30 valpromide periods and 30 placebo periods). Cosinor analysis of HR and MA data revealed a difference in amplitude (P = 0.037, analysis of variance, one-tailed test). The clinical sleep study shows that the duration of sleep was greater with valpromide than with placebo (P = 0.007, one-tailed test). Similarly, evaluation of the quality of sleep by patients themselves showed valpromide to be superior to placebo (P = 0.045, one-tailed test). The results of analysis of the Spiegel questionnaire also confirm the superiority of valpromide over placebo. Safety and compliance were comparable for the active drug and the placebo. In conclusion, the relatively small sample size requires cautious interpretation of this study. Nevertheless, these initial results show a definite effect of valpromide on a biological rhythm that leads one to suppose that it may be effective through a 'synchronizing' effect.

Circadian rhythm of parasuicide in relation to violence of method and concomitant mental disorder

Circadian occurrence of parasuicide was evaluated in relation to sex, violence of parasuicide method and psychiatric diagnosis. In all, 457 consecutive episodes of parasuicide were recruited during a 5-year period. Complete data for time of parasuicide, parasuicide method, parasuicide recurrency and psychiatric diagnosis (ICD-9) were available for 304 subjects. Parasuicide methods were classified into two groups depending on the violence of the method. Parasuicide occurred significantly more often in the afternoon and evening hours for both men and women, for both violent and non-violent methods, both in first-even cases and repeaters, and in the following diagnostic groups: organic mental and psychoactive substance disorders, neurotic disorders and personality disorders. The data support the hypothesis of a circadian rhythmicity of parasuicide, showing an area of chronobiological risk in the afternoon and early evening hours.

Sleep and heart rate circadian rhythm in depression: the necessity to separate

In order to determine whether a decrease in the amplitude of heart rate circadian rhythm (HRCR) could represent a chronobiological marker of depression and to answer the question of the possible role of this alteration in the reduction of rapid eye movement sleep latency (REM latency) observed in depression, 22 major depressed patients (DSM III R, Montgomery Asberg Depression Rating Scale) and 11 healthy volunteers entered this study, which included the simultaneous recordings of sleep and heart rate (HR) during two consecutive 24-h periods. After a smoothing procedure, the HR data, obtained from a portable device providing mean HR/min, were computed with the single cosinor method for the classical circadian parameters. We also determined the cosinor fitting index (CFI = percentage to fit). When studied as a single group and compared to the control group, the depressed patients, who had difficulties in sleep initiation and whose REM latencies were within normal limits, were characterized by a dampening of the amplitude HRCR and of the day-night HR difference. This was linked to a lesser increase of HR during the day and a reduced decrease at night. No significant phase modification of HRCR was observed in this group of depressives. In a second step, the depressives were divided into a depressed arrhythmic group (DAG, CFI < 50%, 11 patients) and a depressed rhythmic group (DRG, CFI > 50%, 11 patients) who did not differ on MADRS scores. Even though the amplitude of HRCR was reduced in the DAG only, with HR higher at night and lower during the daytime than for controls, both DAG and DRG significantly differed from controls on those items related to sleep induction and maintenance. In the DAG only was there a tendency toward a reduction of total sleep time and REM sleep time while REM latency was normal. This finding suggests that sleep problems are not responsible for the differences in HRCR parameters between the two depressed groups. These differences in HR and sleep parameters between two groups of similarly depressed patients also suggest that a short REM latency does not systematically imply a disruption of all circadian clocks.