Effects of acute simulated microgravity on nocturnal sleep, daytime vigilance, and psychomotor performance: comparison of horizontal and 6 degrees head-down bed rest

Differential Functional Changes in Visual Performance during Acute Exposure to Microgravity Analogue and Their Potential Links with Spaceflight-Associated Neuro-Ocular Syndrome

BACKGROUND

Spaceflight-Associated Neuro-Ocular Syndrome (SANS) is a complex pathology threatening the health of astronauts, with incompletely understood causes and no current specific functional diagnostic or screening test. We investigated the use of the differential performance of the visual system (central vs. perimacular visual function) as a candidate marker of SANS-related pathology in a ground-based microgravity analogue.

METHODS

We used a simple reaction time (SRT) task to visual stimuli, presented in the central and perimacular field of view, as a measure of the overall performance of the visual function, during acute settings (first 10 min) of vertical, bed rest (BR), -6°, and -15° head-down tilt (HDT) presentations in healthy participants (n = 8). We built dose-response models linking the gravitational component to SRT distribution parameters in the central vs. perimacular areas.

RESULTS

Acute exposure to microgravity induces detectable changes between SRT distributions in the perimacular vs. central retina (increased mean, standard deviation, and tau component of the ex-Gaussian function) in HDT compared with vertical presentation.

CONCLUSIONS

Functional testing of the perimacular retina might be beneficial for the earlier detection of SANS-related ailments in addition to regular testing of the central vision. Future diagnostic tests should consider the investigation of the extra-macular areas, particularly towards the optic disc.

Effect of 60 days of head down tilt bed rest on amplitude and phase of rhythms in physiology and sleep in men

Twenty-four-hour rhythms in physiology and behaviour are shaped by circadian clocks, environmental rhythms, and feedback of behavioural rhythms onto physiology. In space, 24 h signals such as those associated with the light-dark cycle and changes in posture, are weaker, potentially reducing the robustness of rhythms. Head down tilt (HDT) bed rest is commonly used to simulate effects of microgravity but how HDT affects rhythms in physiology has not been extensively investigated. Here we report effects of -6° HDT during a 90-day protocol on 24 h rhythmicity in 20 men. During HDT, amplitude of light, motor activity, and wrist-temperature rhythms were reduced, evening melatonin was elevated, while cortisol was not affected during HDT, but was higher in the morning during recovery when compared to last session of HDT. During recovery from HDT, time in Slow-Wave Sleep increased. EEG activity in alpha and beta frequencies increased during NREM and REM sleep. These results highlight the profound effects of head-down-tilt-bed-rest on 24 h rhythmicity.

The impact of a short-period head-down tilt on executive function in younger adults

Microgravity has been shown to be a significant stressor on the cardiovascular system and the brain due to the redistribution of fluids that occurs in the absence of gravitational force, but there is scarce literature surrounding the effects of microgravity on cerebral hemodynamics and cognition. Understanding the early effects that simulated gravity has on cognitive function is essential for developing proper physical and cognitive countermeasures to assure safe and effective cognitive/decisions making while astronauts prepare for the initial launch or when they arrive in a microgravity environment. Therefore, this study aims to determine how an acute simulation of microgravity would alter cerebral oxygenation and executive functions. Sixty-five young healthy participants (22 ± 6 years, 21 females) completed a thirty (30) minute horizontal (0⁰ tilt) followed by a 90-min - 6° head-down-tilt (HDT) protocol. Cerebral oxygenation in the prefrontal cortex was monitored throughout the testing session using near-infrared spectroscopy. Cognition was also measured using a computerized Stroop Task. Our results demonstrate that cerebral oxygenation was higher during HDT compared to the horizontal supine position (9.11 ± 1.3 vs. 7.51 ± 1.8, p = 0.02). For the cognitive results, the non-executive performance of the Stroop task remained stable during HDT (652.46 ± 19.3 vs. 632.49 ± 14.5, p = 0.09). However, reaction time during the executive task performance was improved after the HDT (1058 ± 195-950 ± 158 ms, p < 0.01). Our results suggest that an acute bout of simulated microgravity can enhance executive functioning.

Evaluation of Vestibular Function in Patients Affected by Obstructive Sleep Apnea Performing Functional Head Impulse Test (fHIT)

PURPOSE

Obstructive sleep apnoea (OSA) is a common disease with significantly related complications. Since a connection between the vestibular nucleus and sleep regulator pathways has been demonstrated, vestibular evaluation in OSA patients was partially studied and none used functional head impulse test (fHIT) for this purpose. This paper aimed at evaluating the vestibular function in patients affected by OSA using fHIT, selecting patients who did not present any other related to cardiovascular, neurological, or metabolic diseases.

PATIENTS AND METHODS

Patients enrolled had a diagnosis of OSA by polysomnography type III and were cataloged according to American Association of Sleep Medicine criteria. Each patient underwent fHIT. Statistical significance was set at 0.05.

RESULTS

A total of 85 patients were enrolled in the study of which 50 had a diagnosis of OSA and were included in the case group, while 35 belonged to the control group. In 88.6% of subjects of the case group was evidenced a vestibular impairment with a substantial difference between the two study groups (p<0.05).

CONCLUSION

The results show that the incidence of vestibular lesions in patients with obstructive sleep apnoea is underestimated and that fHIT can identify these lesions early.

Long-Term Bed Rest Delays the Circadian Phase of Core Body Temperature

Spaceflight can be associated with sleep loss and circadian misalignment as a result of non-24 h light-dark cycles, operational shifts in work/rest cycles, high workload under pressure, and psychological factors. Head-down tilt bed rest (HDBR) is an established model to mimic some of the physiological and psychological adaptions observed in spaceflight. Data on the effects of HDBR on circadian rhythms are scarce. To address this gap, we analyzed the change in the circadian rhythm of core body temperature (CBT) in two 60-day HDBR studies sponsored by the European Space Agency [n = 13 men, age: 31.1 ± 8.2 years (M ± SD)]. CBT was recorded for 36 h using a non-invasive and validated dual-sensor heatflux technology during the 3rd and the 8th week of HDBR. Bed rest induced a significant phase delay from the 3rd to the 8th week of HDBR (16.23 vs. 16.68 h, p = 0.005, g = 0.85) irrespective of the study site (p = 0.416, g = −0.46), corresponding to an average phase delay of about 0.9 min per day of HDBR. In conclusion, long-term bed rest weakens the entrainment of the circadian system to the 24-h day. We attribute this effect to the immobilization and reduced physical activity levels associated with HDBR. Given the critical role of diurnal rhythms for various physiological functions and behavior, our findings highlight the importance of monitoring circadian rhythms in circumstances in which gravity or physical activity levels are altered.

Sleep in Isolated, Confined, and Extreme (ICE): A Review on the Different Factors Affecting Human Sleep in ICE

The recently renewed focus on the human exploration of outer space has boosted the interest toward a variety of questions regarding health of astronauts and cosmonauts. Among the others, sleep has traditionally been considered a central issue. To extend the research chances, human sleep alterations have been investigated in several analog environments, called ICEs (Isolated, Confined, and Extreme). ICEs share different features with the spaceflight itself and have been implemented in natural facilities and artificial simulations. The current paper presents a systematic review of research findings on sleep disturbances in ICEs. We looked for evidence from studies run in polar settings (mostly Antarctica) during space missions, Head-Down Bed-Rest protocols, simulations, and in a few ICE-resembling settings such as caves and submarines. Even though research has shown that sleep can be widely affected in ICEs, mostly evidencing general and non-specific changes in REM and SWS sleep, results show a very blurred picture, often with contradictory findings. The variable coexistence of the many factors characterizing the ICE environments (such as isolation and confinement, microgravity, circadian disentrainment, hypoxia, noise levels, and radiations) does not provide a clear indication of what role is played by each factor per se or in association one with each other in determining the pattern observed, and how. Most importantly, a number of methodological limitations contribute immensely to the unclear pattern of results reported in the literature. Among them, small sample sizes, small effect sizes, and large variability among experimental conditions, protocols, and measurements make it difficult to draw hints about whether sleep alterations in ICEs do exist due to the specific environmental characteristics, and which of them plays a major role. More systematic and cross-settings research is needed to address the mechanisms underlying the sleep alterations in ICE environments and possibly develop appropriate countermeasures to be used during long-term space missions.

Sleep Is Compromised in -12° Head Down Tilt Position

Recent studies are elucidating the interrelation between sleep, cranial perfusion, and cerebrospinal fluid (CSF) circulation. Head down tilt (HDT) as a simulation of microgravity reduces cranial perfusion. Therefore, our aim was to assess whether HDT is affecting sleep (clinicaltrials.gov; identifier NCT 02976168). 11 male subjects were recruited for a cross-over designed study. Each subject participated in two campaigns each comprising 3 days and 2 nights. Intervention started on the second campaign day and consisted of maintenance of horizontal position or −12° HDT for 21 h. Ultrasound measurements were performed before, at the beginning and the end of intervention. Polysomnographic measurements were assessed in the second night which was either spent in horizontal posture or at −12° HDT. Endpoints were sleep efficiency, sleep onset latency, number of sleep state changes and arousals, percentages of N3, REM, light sleep stages and subjective sleep parameters. N3 and REM sleep reduced by 25.6 and 19.1 min, respectively (P = 0.002, g = -0.898; P = 0.035, g = -0.634) during −12° HDT. Light sleep (N1/2) increased by 33.0 min at −12° HDT (P = 0.002, g = 1.078). On a scale from 1 to 9 subjective sleep quality deteriorated by 1.3 points during −12° HDT (P = 0.047, g = -0.968). Ultrasonic measurement of the venous system showed a significant increase of the minimum (P = 0.009, P < 0.001) and maximum (P = 0.004, P = 0.002) cross-sectional area of the internal jugular vein at −12° HDT. The minimum cross-sectional area of the external jugular vein differed significantly between conditions over time (P = 0.001) whereas frontal skin tissue thickness was not significantly different between conditions (P = 0.077, P = 0.811). Data suggests venous congestion at −12° HDT. Since subjects felt comfortable with lying in −12° HDT under our experimental conditions, this posture only moderately deteriorates sleep. Obviously, the human body can almost compensate the several fold effects of gravity in HDT posture like an affected CSF circulation, airway obstruction, unusual patterns of propioception and effects on the cardiovascular system.

Bed Rest and Hypoxic Exposure Affect Sleep Architecture and Breathing Stability

Objective: Despite over 50 years of research on the physiological effects of sustained bed rest, data characterizing its effects on sleep macrostructure and breathing stability in humans are scarce. This study was conducted to determine the effects of continuous exposure to hypoxia and sustained best rest, both individually and combined, on nocturnal sleep and breathing stability.

Methods: Eleven participants completed three randomized, counter-balanced, 21-days trials of: (1) normoxic bed rest (NBR, P_IO₂ = 133.1 ± 0.3), (2) hypoxic ambulatory confinement (HAMB, P_IO₂ = 90.0 ± 0.4) and (3) hypoxic bed rest (HBR, P_IO₂ = 90.0 ± 0.4; ~4,000 m equivalent altitude). Full objective polysomnography was performed at baseline, on Night 1 and Night 21 in each condition.

Results: In NBR Night 1, more time was spent in light sleep (10 ± 2%) compared to baseline (8 ± 2%; p = 0.028); Slow-wave sleep (SWS) was reduced from baseline in the hypoxic-only trial by 18% (HAMB Night 21, p = 0.028) and further reduced by 33% (HBR Night 1, p = 0.010), and 36% (HBR Night 21, p = 0.008) when combined with bed rest. The apnea-hypopnea index doubled from Night 1 to Night 21 in HBR (32–62 events·h⁻¹) and HAMB (31–59 events·h⁻¹; p = 0.002). Those who experienced greatest breathing instability from Night 1 to Night 21 (NBR) were correlated to unchanged or higher (+1%) night SpO₂ concentrations (R² = 0.471, p = 0.020).

Conclusion: Bed rest negatively affects sleep macrostructure, increases the apnea-hypopnea index, and worsens breathing stability, each independently exacerbated by continuous exposure to hypoxia.

Analogs of microgravity: head-down tilt and water immersion

This article briefly reviews the fidelity of ground-based methods used to simulate human existence in weightlessness (spaceflight). These methods include horizontal bed rest (BR), head-down tilt bed rest (HDT), head-out water immersion (WI), and head-out dry immersion (DI; immersion with an impermeable elastic cloth barrier between subject and water). Among these, HDT has become by far the most commonly used method, especially for longer studies. DI is less common but well accepted for long-duration studies. Very few studies exist that attempt to validate a specific simulation mode against actual microgravity. Many fundamental physical, and thus physiological, differences exist between microgravity and our methods to simulate it, and between the different methods. Also, although weightlessness is the salient feature of spaceflight, several ancillary factors of space travel complicate Earth-based simulation. In spite of these discrepancies and complications, the analogs duplicate many responses to 0 G reasonably well. As we learn more about responses to microgravity and spaceflight, investigators will continue to fine-tune simulation methods to optimize accuracy and applicability.

The separate and combined effects of hypoxia and sustained recumbency/inactivity on sleep architecture

PURPOSE

The objective was to determine the separate and combined effects of hypoxia and inactivity/unloading on sleep architecture during a 10-day period of confinement.

METHODS

Ten subjects participated in three 10-day trials in random order: hypoxic ambulatory (HAMB), hypoxic bedrest (HBR), and normoxic bedrest (NBR). During the HAMB and HBR trials, subjects were confined to a hypoxic facility. The hypoxia profile was: simulated altitude of 2,990 m on day 1, 3,380 m on day 2, and 3,881 m on day 3. In the NBR and HBR trials, subjects maintained a horizontal position throughout the confinement period. During each trial, sleep polysomnography was conducted one night prior to (baseline; altitude of facility is 940 m) and on the first (NT1, altitude 2,990 m) and tenth (NT10, altitude 3,881 m) night of the 10-day intervention.

RESULTS

Average time in sleep stage 1 decreased from NT1 to NT10 irrespective of trial. Overall incidence and time spent in periodic breathing increased from NT1 to NT10 in both HAMB and HBR. During NT1, both HAMB and HBR reduced slow-wave sleep and increased light sleep, whereas NBR and HBR increased the number of awakenings/night. There were fewer awakenings during HAMB than NBR.

CONCLUSIONS

Acute exposure to both hypoxia and bedrest (HBR) results in greater sleep fragmentation due to more awakenings attributed to bedrest, and lighter sleep as a result of reduced slow wave sleep caused by the hypoxic environment.

Influence of body position on cortical pain-related somatosensory processing: an ERP study

BACKGROUND

Despite the consistent information available on the physiological changes induced by head down bed rest, a condition which simulates space microgravity, our knowledge on the possible perceptual-cortical alterations is still poor. The present study investigated the effects of 2-h head-down bed rest on subjective and cortical responses elicited by electrical, pain-related somatosensory stimulation.

METHODOLOGY/PRINCIPAL FINDINGS

Twenty male subjects were randomly assigned to two groups, head-down bed rest (BR) or sitting control condition. Starting from individual electrical thresholds, Somatosensory Evoked Potentials were elicited by electrical stimuli administered randomly to the left wrist and divided into four conditions: control painless condition, electrical pain threshold, 30% above pain threshold, 30% below pain threshold. Subjective pain ratings collected during the EEG session showed significantly reduced pain perception in BR compared to Control group. Statistical analysis on four electrode clusters and sLORETA source analysis revealed, in sitting controls, a P1 component (40-50 ms) in the right somatosensory cortex, whereas it was bilateral and differently located in BR group. Controls' N1 (80-90 ms) had widespread right hemisphere activation, involving also anterior cingulate, whereas BR group showed primary somatosensory cortex activation. The P2 (190-220 ms) was larger in left-central locations of Controls compared with BR group.

CONCLUSIONS/SIGNIFICANCE

Head-down bed rest was associated to an overall decrease of pain sensitivity and an altered pain network also outside the primary somatosensory cortex. Results have implications not only for astronauts' health and spaceflight risks, but also for the clinical aspects of pain detection in bedridden patients at risk of fatal undetected complications.