Sleep and circadian disruption during human spaceflight: mechanisms, operational consequences, and countermeasures

1 Introduction

Exploration-class missions will expose crews to prolonged isolation, confinement, operational autonomy, altered gravitational environments, and schedules that may be poorly aligned with terrestrial circadian timing. Within this risk landscape, sleep is both a biological requirement and a determinant of mission reliability. Insufficient or mistimed sleep can impair vigilance, decision-making, mood regulation, and psychomotor performance at precisely the moments when crews must manage complex systems with limited external support.

Sleep problems have been reported throughout the history of human spaceflight. Early shuttle observations identified disrupted and shortened sleep, while later studies on shuttle, Mir, and the International Space Station (ISS) documented circadian phase shifts, sleep deficiency, and frequent use of hypnotic medication (Santy et al., 1988; Gundel et al., 1997; Monk et al., 1998; Barger et al., 2014). Analog studies such as Mars500 have added evidence that isolation, restricted activity, and schedule drift can disturb sleep-wake timing even without microgravity (Basner et al., 2013). Together, these findings indicate that sleep disruption arises from the interaction of the mission environment, operational demands, individual vulnerability, and adaptive physiology.

Terminology requires care. Short sleep duration, poor sleep quality, circadian misalignment, and fatigue do not by themselves establish a clinical sleep disorder. Insomnia disorder requires persistent sleep difficulty despite adequate opportunity, associated daytime impairment, and diagnostic assessment. Similarly, circadian rhythm sleep-wake disorders require evidence that symptoms result from misalignment between endogenous timing and the required sleep schedule (Pavlova and Latreille, 2019; Liu et al., 2022). Most spaceflight studies report sleep disturbance or deficiency rather than formal diagnoses. This review therefore uses disorder-specific terms only when supported by diagnostic evidence and otherwise refers to sleep or circadian disruption.

The objectives are to synthesize the physiological and operational mechanisms that disturb astronaut sleep; evaluate consequences for health and performance; compare established and emerging countermeasures; and propose a research and operational agenda for lunar and Martian missions. The central argument is that effective mitigation will require an individualized, multimodal system that protects sleep opportunity while dynamically coordinating lighting, schedules, habitat conditions, monitoring, and medication use.

2 Review approach

This article is a focused narrative review of peer-reviewed human spaceflight, space-analog, sleep, and circadian-medicine literature published through January 2026. The evidence base was organized around four questions: how sleep is regulated; which spaceflight exposures disrupt that regulation; how sleep deficiency affects health and performance; and which countermeasures are supported by operational or experimental evidence. Human spaceflight evidence was prioritized when available. Analog, terrestrial, animal, and mechanistic studies were used to interpret pathways or emerging interventions, with their lower direct applicability to astronauts stated explicitly. Because the review was not designed as a systematic review, it does not claim exhaustive retrieval or formal risk-of-bias grading. Its purpose is critical integration of the available evidence and identification of testable priorities.

3 Biological regulation of sleep

3.1 Circadian timing and homeostatic sleep pressure

Human sleep timing is commonly described by the interaction of two processes. The homeostatic process increases sleep pressure during wakefulness and dissipates it during sleep. The circadian process generates an approximately 24-h rhythm that promotes wakefulness and sleep at different biological phases. Stable sleep occurs when adequate homeostatic pressure coincides with the circadian sleep window; misalignment can produce difficulty initiating sleep, premature awakening, or excessive sleepiness despite substantial sleep debt (Borbely and Achermann, 1999; Kramer et al., 2022).

The suprachiasmatic nucleus (SCN) of the hypothalamus is the principal circadian pacemaker. Light detected by intrinsically photosensitive retinal ganglion cells provides the dominant environmental timing signal, enabling the SCN to synchronize clocks throughout the body. Molecular clocks are generated by transcriptional-translational feedback loops involving proteins encoded by genes such as CLOCK, BMAL1, PER, and CRY. These rhythms coordinate sleep-wake timing with temperature, endocrine signaling, metabolism, immunity, and other physiological functions (McGlashan et al., 2018; Overeem et al., 2021; Kramer et al., 2022).

Melatonin is secreted by the pineal gland under circadian control and provides a biological-night signal. Its timing is more informative than a simple characterization as a sleep hormone: appropriately timed melatonin can shift circadian phase and modestly facilitate sleep, whereas mistimed administration may be ineffective or counterproductive. Melatonin also interacts with inflammatory and immune processes, illustrating why circadian disruption may have consequences beyond subjective sleep quality (Radogna et al., 2010; Ding et al., 2024).

3.2 Sleep architecture

Sleep alternates between non-rapid eye movement (NREM) and rapid eye movement (REM) states. NREM sleep progresses from light sleep to slow-wave sleep and contributes to physiological restoration, memory processes, and regulation of sleep pressure. REM sleep is characterized by cortical activation, muscle atonia, and vivid dreaming and contributes to emotional and memory processing. Changes in total sleep time alone may therefore conceal alterations in architecture. Polysomnography remains the reference standard, but its operational burden limits routine deployment in space. Wearables and actigraphy are more feasible yet require validation because proprietary algorithms may misclassify sleep stages, especially under unusual movement and environmental conditions (Nikkonen et al., 2024).

4 Sleep and circadian disruption in space

4.1 Prevalence and phenotype

Across shuttle and ISS studies, astronauts have commonly obtained approximately 6–6.5 h of sleep per 24 h, below the sleep opportunity generally required for sustained alertness and health (Barger et al., 2014; Wu et al., 2018). Circadian misalignment has been associated with still shorter sleep and greater medication use (Flynn-Evans et al., 2016). During 6-month ISS missions, sleep deficiency was associated with poorer neurobehavioral performance and higher stress, and longer mission duration did not reliably normalize sleep (Jones et al., 2022). Recent observations also suggest that sleep architecture may change across long-duration flight, with recovery of some REM measures occurring alongside reduced NREM sleep (Piltch et al., 2025).

These observations support a phenotype of insufficient, fragmented, and sometimes mistimed sleep. They do not establish that most astronauts have insomnia disorder. High hypnotic use, difficulty sleeping, and curtailed sleep opportunity may reflect different mechanisms and require different interventions. A major limitation of the field is the absence of a harmonized diagnostic framework combining clinical interviews, validated questionnaires, objective sleep measures, circadian phase markers, and operational context.

4.2 Environmental and operational drivers

The ISS completes an orbit approximately every 90 min, exposing crew to multiple external light-dark transitions each day. Interior lighting and window exposure, rather than orbital sunrise itself, determine retinal light input; however, poorly timed or insufficient light can weaken circadian entrainment. Schedule shifts, night operations, pre-sleep screen exposure, and irregular wake times further destabilize circadian phase (Dijk et al., 2001; Wu et al., 2018).

Workload is an equally important driver. On the ISS, higher workload has been associated with shorter sleep duration, while operational events can intrude into scheduled sleep periods (Jones et al., 2022). A schedule may allocate nominal sleep time yet fail to provide genuine sleep opportunity when crews must complete delayed tasks, prepare for the next day, communicate with the ground, or recover from stress. Noise from life-support systems, uncomfortable temperature, light leakage, limited privacy, and sleeping-restraint discomfort add arousal and fragmentation. These factors are modifiable and should not be treated as unavoidable features of spaceflight.

Microgravity may influence sleep through fluid shifts, vestibular adaptation, altered movement, respiratory mechanics, and changes in the physical cues associated with posture. The direction and magnitude of its independent effect remain uncertain because microgravity is inseparable from concurrent schedule, habitat, and operational changes in orbital studies. Claims that reduced gravity simply lowers physiological sleep need are not sufficiently established. Future studies require concurrent ground controls, standardized sleep opportunity, and mechanistic measurements to separate gravity-related effects from mission context.

4.3 Biological and psychosocial pathways

Spaceflight can alter metabolism, immune regulation, neuroendocrine signaling, and microbial composition, all of which interact bidirectionally with sleep and circadian timing. Circadian disruption can impair glucose regulation and promote inflammatory signaling; conversely, metabolic and inflammatory changes may disturb sleep (Maury, 2019; Kramer et al., 2022; Tomaraei et al., 2025). Spaceflight-associated microbiome changes provide another plausible pathway, but current evidence does not demonstrate that microbiome modification can prevent astronaut sleep disturbance (Morrison et al., 2021).

Psychological stress, anticipatory arousal, interpersonal tension, and the loss of normal environmental cues may contribute to difficulty sleeping. Hyperarousal is central to terrestrial insomnia and may become self-reinforcing: concern about sleep and next-day performance increases arousal, which further delays sleep (Van Someren, 2021). Individual vulnerability is substantial and cannot be inferred solely from astronaut selection or physical fitness. Figure 1 integrates these exposures, mediating processes, outcomes, modifiers, and intervention points.

5 Consequences for health and mission performance

The most immediate mission-relevant consequences of sleep deficiency are reduced vigilant attention, slower psychomotor responses, impaired executive control, and increased lapses. Astronauts sleeping fewer than 6 h have shown degraded neurobehavioral performance, and simulated missions with 5-h sleep opportunities produced worse performance than 8-h opportunities (Flynn-Evans et al., 2020; Jones et al., 2022). Deficits may be especially dangerous during routine or monotonous tasks, when low stimulation permits attention to drift, as well as during emergencies requiring rapid integration of unfamiliar information.

Sleep loss also affects mood, stress reactivity, motivation, and interpersonal functioning. In small, isolated crews, these effects can propagate through team dynamics and decision-making. Circadian misalignment independently compounds risk because a task performed during the biological night may be impaired even after apparently adequate sleep. Operational models should therefore consider sleep duration, prior wake, circadian phase, workload, and individual response rather than relying on a single fatigue indicator.

Long-term health implications are biologically plausible but less directly quantified in astronauts. Terrestrial evidence links chronic sleep and circadian disruption with cardiometabolic, inflammatory, and mental-health outcomes (Maury, 2019; Walker et al., 2020; Liu et al., 2022). Extrapolation to highly selected astronaut cohorts must remain cautious, particularly because mission duration, countermeasures, and post-flight recovery differ from terrestrial shift-work exposure. Nevertheless, exploration missions may extend exposure while reducing access to specialist care, making prevention preferable to treatment.

6 Countermeasures

6.1 Protecting sleep opportunity and controlling workload

The foundation of fatigue risk management is sufficient protected sleep opportunity. No lighting system or medication can reliably compensate for chronic schedule-driven sleep restriction. Mission planners should protect stable sleep windows, limit non-critical intrusions, monitor cumulative workload, and build recovery opportunities around high-risk operations. Strategic naps may restore acute alertness, but duration and timing should minimize sleep inertia and avoid delaying the main sleep episode. Autonomous missions will also require crew-centered scheduling tools that can resolve task conflicts locally (Saint-Guillain et al., 2023).

6.2 Timed light and circadian phase management

Light is the most powerful practical circadian countermeasure. Its effect depends on intensity, spectrum, duration, and—most importantly—biological timing. Short-wavelength-enriched light can increase alertness and suppress melatonin, whereas lower-intensity and short-wavelength-reduced light before sleep supports biological night. The ISS Solid-State Lighting Assembly enables variable lighting, but shared habitats and individual differences complicate delivery. Future systems should combine personal light exposure, sleep timing, and estimated circadian phase to provide individualized prescriptions while preventing contradictory light signals.

6.3 Habitat and behavioral measures

Environmental optimization is low risk and broadly applicable. Sleeping quarters should minimize noise, vibration, light leakage, temperature variability, and unnecessary communication. Restraints should permit a comfortable neutral posture without pressure points. Pre-sleep routines, controlled caffeine timing, regular exercise, and psychologically safe opportunities to disengage from work can reduce arousal. These measures are often described as basic sleep hygiene, but in spacecraft they depend on engineering and operational design rather than individual discipline alone.

6.4 Pharmacological approaches

Hypnotics have been widely used during spaceflight, yet observational data indicate that faster sleep onset does not necessarily translate into longer or better-quality sleep (Barger et al., 2014). Residual sedation, impaired balance, altered cognition after awakening, drug-drug interactions, and the need to respond immediately to alarms are important safety considerations. Pharmacokinetics may also change during spaceflight, although evidence is insufficient to assume a uniform reduction in absorption. Medication protocols should therefore define indication, timing, minimum recovery interval, prohibited combinations, individual ground testing, and emergency-response constraints.

Melatonin differs from conventional hypnotics because it can act as a circadian phase-shifting signal as well as a sleep-promoting agent. Its effectiveness depends on timing relative to endogenous circadian phase and should be coordinated with light exposure. Pharmacological countermeasures should remain adjuncts to protected sleep opportunity and schedule management, not substitutes for them.

6.5 Emerging and adaptive approaches

Microbiome-directed, dietary, and traditional medicine approaches are emerging but remain preliminary. Animal evidence that Gastrodia elata may ameliorate cognitive effects of circadian disruption does not establish efficacy or safety in astronauts (Huang et al., 2021). Similarly, correlations among spaceflight, microbial composition, and sleep do not yet justify microbiome therapy. Candidate interventions should undergo compositional standardization, toxicology, interaction testing, controlled terrestrial trials, analog evaluation, and staged flight validation before operational use.

The most promising systems-level development is adaptive fatigue-risk management. Wearable-derived sleep estimates, light exposure, workload, cognitive testing, and self-report could inform personalized schedules and countermeasures (Zong et al., 2025). Artificial intelligence may assist with pattern recognition and scheduling, but recommendations must remain interpretable, robust to sensor failure, privacy-preserving, and usable without ground connectivity. Automation should support crew and flight-surgeon decisions rather than obscure uncertainty. Table 1 summarizes the evidence, value, and limitations of the principal countermeasure classes.

TABLE 1. Countermeasures for sleep and circadian disruption during spaceflight.

6.6 Precision monitoring, personalized chronotherapy, and closed-loop control

Multi-omics approaches could integrate longitudinal transcriptomic, epigenomic, proteomic, metabolomic, and microbiome measurements with sleep and circadian phenotyping to identify individual response signatures and pathways linking circadian disruption to immune, metabolic, and neurobehavioral outcomes. In astronaut cohorts, this should be treated as a research framework rather than a validated countermeasure. Small sample sizes, repeated-measures dependence, limited in-flight sampling, batch effects, and concurrent exposures such as microgravity, radiation, workload, diet, and stress complicate causal interpretation. Pre-flight baselines, sampling standardized to sleep-wake and circadian phase, harmonized processing, and within-person analyses would be essential (Kramer et al., 2022; Morrison et al., 2021).

Personalized chronotherapy would time light exposure, melatonin, caffeine, exercise, naps, and demanding tasks according to estimated circadian phase, chronotype, recent sleep history, and prior individual response. Wearable systems could support this approach by combining actigraphy with personal light exposure, heart rate and heart-rate variability, distal skin temperature, and other feasible physiological signals. These measures should not be treated as interchangeable with polysomnography or direct circadian phase markers; their algorithms require in-flight validation, uncertainty reporting, data-quality checks, and explicit handling of sensor failure (Nikkonen et al., 2024; Zong et al., 2025).

At habitat level, closed-loop environmental control systems could combine monitored state, mission schedules, and crew preferences to adjust personal lighting, temperature, acoustic masking, and the timing of non-critical alerts. Crew override, transparent decision rules, conservative safety limits, privacy protection, and robust offline operation would remain mandatory. Such systems should avoid automated clinical diagnosis and undergo staged testing in terrestrial laboratories, analog missions, and flight. Their value should be assessed against sleep, circadian, performance, acceptability, and failure-mode outcomes rather than prediction accuracy alone.

7 Research priorities for exploration-class missions

Progress is limited less by a shortage of proposed countermeasures than by heterogeneity in measurement and study design. A core sleep dataset should be adopted across missions: scheduled and actual sleep opportunity; actigraphy or validated wearable measures; sleep diary and standardized symptoms; personal light exposure; workload and mission events; medication and caffeine use; and repeated neurobehavioral performance. Selected campaigns should include polysomnography and circadian phase markers to validate operational proxies. Multi-omics sampling should be time-stamped relative to sleep, meals, exercise, medication, and circadian phase so that molecular variation is not misinterpreted as a mission effect. Studies of personalized chronotherapy and closed-loop habitat control should use predefined safety constraints, crew override, and prospective comparisons with non-adaptive protocols.

Clinical terminology should be harmonized. Studies should distinguish insufficient sleep opportunity, circadian misalignment, insomnia symptoms, insomnia disorder, sleep-disordered breathing, and medication-related sleep changes. Diagnostic conclusions require appropriate criteria rather than inference from short sleep or hypnotic use. Pre-flight baselines and post-flight recovery measurements should use comparable timing and instruments.

Future research must also address diversity and individual variability. Astronaut cohorts are small, but pooled international protocols can improve statistical power. Sex, chronotype, age, prior sleep history, genetic susceptibility, and response to light or sleep restriction may modify risk. Reporting these factors, while protecting privacy, will be essential for personalized countermeasures.

Finally, countermeasures should be evaluated as integrated packages over mission-relevant durations. The most informative studies will compare protected sleep and adaptive scheduling, personalized light, environmental control, and conservative pharmacology under realistic autonomy, workload, and communication delay. Lunar and Martian habitats offer an opportunity to treat lighting, acoustics, private sleep volume, and monitoring infrastructure as parts of the life-support architecture from the outset.

8 Discussion

The evidence supports a consistent conclusion: sleep and circadian disruption in space are multicausal operational hazards, not isolated symptoms with a single remedy. The same observed short sleep can arise from curtailed opportunity, circadian mistiming, environmental disturbance, hyperarousal, medication effects, or an underlying sleep disorder. Effective mitigation therefore depends on identifying the dominant mechanism in each astronaut and mission phase.

Current countermeasures have complementary strengths. Protected sleep opportunity and environmental control address preventable operational causes. Timed light and melatonin can shift circadian phase. Naps and alertness strategies manage acute risk. Hypnotics may have a limited role when non-pharmacological measures are insufficient, but their residual effects matter in a safety-critical setting. Emerging dietary, microbiome, and traditional medicine approaches are hypotheses requiring staged validation, not operational alternatives to established fatigue management.

The review has limitations. It is narrative rather than systematic, so selection bias cannot be excluded and study quality was not formally graded. Human spaceflight studies involve small samples and heterogeneous missions; analog studies reproduce only subsets of spaceflight exposures; and mechanistic or animal findings may not translate to astronauts. Several recent reviews synthesize overlapping literature, which increases the risk that weak primary evidence is repeated with greater apparent certainty. Claims were therefore weighted toward direct human spaceflight findings and qualified where evidence is indirect or contested.

For long-duration missions, sleep should be managed as a mission-critical system variable. A closed-loop approach would continuously integrate sleep opportunity, circadian timing, light exposure, workload, performance, symptoms, and medication use; recommend transparent adjustments; and preserve crew autonomy. The objective is not to eliminate normal variation in sleep but to maintain adequate recovery and operational readiness while detecting clinically significant problems early. Wearable physiological biomarkers could provide continuous operational inputs, while multi-omics measurements would be better reserved for periodic research and validation because sampling and interpretation are more demanding. Environmental outputs could include individualized lighting, temperature, acoustic conditions, scheduling, and alert timing, with transparent recommendations and crew override.

9 Conclusion

Spaceflight disrupts sleep through interacting environmental, operational, biological, and psychosocial pathways. The resulting sleep deficiency and circadian misalignment can degrade vigilance, psychomotor performance, mood, and team functioning, while the prevalence of clinically defined sleep disorders remains uncertain. No single countermeasure is sufficient. Exploration-class missions require protected sleep opportunity, adaptive workload management, individualized circadian lighting, an optimized sleep environment, validated monitoring, and cautious pharmacology. Standardized diagnostics and longitudinal, multinational studies are needed to distinguish symptoms from disorders and to determine which integrated countermeasure packages remain effective during lunar and Martian missions.

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Footnotes

Edited by: Joseph N. Pelton, International Space University, United States

Reviewed by: Luis Rafael Moscote-Salazar, AV Healthcare Innovators, LLC, United States