Long voyages beyond Earth will demand clear thinking even when sleep is interrupted by alarms, urgent cargo work, or changing mission plans. A four-month isolation experiment found that healthy crew members handled brief sleep interruptions with relatively stable nervous system activity, while a full night without sleep produced the strongest physical strain.
The 2026 study, published in Frontiers in Human Neuroscience, followed six people during the SIRIUS-19 space analogue mission. Researchers recorded sleep, pulse rate and changes in the timing between heartbeats across eight selected nights. One night included complete sleep loss, while two others included planned awakenings.
The results offer a small but useful look at how the body responds when sleep is disturbed during a long period of confinement. Crew members generally slept well on undisturbed nights inside the habitat. Broken sleep reduced sleep quality, yet sustained wakefulness produced the clearest rise in activity within the autonomic nervous system, which controls functions such as heart rate and blood pressure without conscious effort.
Six crew members across eight nights
The experiment included three women and three men between 28 and 44 years old. Four were Russian and two were American. Their roles included a commander who had flown to the International Space Station, a flight engineer, a crew physician and three researchers with earlier experience in isolation studies.
Medical screening found no major heart conditions or known sleep disorders among the six healthy participants. Their experience and physical fitness helped them manage a demanding research schedule, although those qualities may also have made them more resistant to sleep loss than a wider group of people.
Researchers monitored eight nights spread across the mission. One measurement occurred before isolation, six took place during confinement and a final night followed the crew’s exit. Inside the habitat, the schedule included three undisturbed nights, two nights of sleep fragmentation and one night of complete sleep loss.
How SIRIUS-19 recreated mission pressure
SIRIUS-19 was a 120-day ground experiment carried out in Moscow at the NEK isolation facility. The international SIRIUS program studies how crews respond to long periods in a sealed habitat. NASA describes the NEK facility as a collection of connected modules designed to create many of the living and working conditions expected during deep-space travel.
Daily life followed a controlled schedule. Artificial lighting supported a regular day and night cycle, with planned light exposure from 7 a.m. until 11 p.m. Meals and physical activity followed mission plans. Crew members lived in restricted quarters and carried out tasks under communication rules intended to resemble an interplanetary expedition.
Earth gravity remained in place, so the experiment focused on confinement and mission-like work. The official SIRIUS-19 booklet describes the wider project as a step toward preparing human crews for deep-space missions through controlled studies on Earth.
Several sleep interruptions were built into the schedule as realistic operational events. On mission day 91, the crew was awakened at 2 a.m. and again at 4 a.m. Lights came on and a radio message announced an approaching supply ship. A second announcement soon canceled the work, allowing the crew to return to bed.
Another test occurred on day 108. Crew members were awakened at 3:15 a.m. and kept awake for one hour while the lights remained on and a disturbing sound played. The full sleep-deprivation test took place on day 54, when the crew stayed awake overnight to simulate unloading a cargo ship.
Broken sleep cut sleep efficiency below 80 percent
Two undisturbed nights during isolation produced more than seven hours of sleep on average. The crew slept for about 421 minutes on mission day 5 and about 431 minutes on day 71. Sleep efficiency reached roughly 92 percent on both nights, meaning that most of the time spent in bed was used for sleep.
Planned awakenings had a clear effect on sleep efficiency. The average fell to 77.4 percent during the night with two brief interruptions. It reached 76.8 percent during the night with the longer awakening. Both values were below the 80 percent level highlighted by the researchers.
Total sleep time also changed significantly across the recorded nights. Crew members averaged about 342 minutes of sleep during the first fragmented night and nearly 395 minutes during the second. The amount of wakefulness rose, especially on the night with two separate alarms.
Sleep stage proportions remained broadly similar across the study. The researchers found no statistically significant differences in the shares of light sleep, deep sleep, or rapid eye movement sleep. The clearest effect of the alarms involved continuity, since crew members spent more of the night awake and used their time in bed less efficiently.
A sleepless night raised autonomic activation
The largest physical response appeared during complete sleep deprivation. Average pulse rate reached 66.4 beats per minute during the sleepless night, the highest value recorded under any condition. Undisturbed isolation nights generally produced lower averages, including 55.8 beats per minute on day 71.
A measure comparing different rhythms within heart rate variability also rose during the night without sleep. Its average value reached 2.3, compared with about 1.3 to 1.5 during several normal isolation nights. Researchers interpreted the rise as a shift toward greater activation of the body’s alert and stress systems.
The study discussion states, “A night of complete sleep deprivation was associated with increased autonomic activation.” The result came from the same six people measured repeatedly, allowing each crew member’s responses under disrupted conditions to be compared with responses on calmer nights.
Subjective sleepiness showed a similar direction, although the result did not reach the study’s threshold for statistical significance. Four of the six crew members felt sleepier in the morning after staying awake all night. Their morning scores increased by an average of 2.5 points on the nine-point Karolinska Sleepiness Scale.
What heart rate variability reveals
Heart rate variability, often shortened to HRV, measures small changes in the time between heartbeats. A healthy heart does not beat at perfectly fixed intervals. The spacing changes as the body responds to breathing, movement, emotional strain and the need for rest.
Two main branches of the autonomic nervous system influence the heart. The sympathetic branch supports alertness and prepares the body for effort. The parasympathetic branch slows activity and supports recovery during quiet periods, especially during healthy sleep.
Researchers examined the LF/HF ratio, which compares low-frequency and high-frequency parts of the heartbeat pattern. A higher value can suggest stronger sympathetic influence or weaker recovery activity under some conditions. During the sleepless night, the higher ratio appeared alongside an increased pulse rate, supporting the study’s interpretation of greater autonomic activation.
HRV requires careful reading because breathing, posture, movement and measurement methods can affect the result. Fragmented sleep can also change breathing patterns. The researchers therefore treated the ratio as one part of a wider group of measurements rather than using it alone to judge the crew’s health.
Portable sleep monitors worked in isolation
Each crew member used a portable medical-grade sleep recorder called SOMNOtouch RESP. Sensors recorded electrical activity from the brain and eyes, helping a trained sleep specialist identify sleep stages. Other sensors tracked the heart, pulse and blood oxygen signals.
Crew members applied the equipment themselves before going to bed. Self-use is valuable inside a remote habitat, where a full sleep laboratory and a team of technicians would take up space and crew time. The same recording setup was used throughout the four-month mission to keep measurements consistent.
Reliable data were collected during normal nights and both fragmentation tests. The device also recorded cardiovascular activity throughout the night of forced wakefulness. Such portable sleep monitoring could help mission doctors watch for rising fatigue during future expeditions, especially when a crew faces repeated alarms or heavy workloads.
The study did not compare the portable unit directly with a complete laboratory sleep system during the mission. Its successful operation still provides evidence that crew members can collect detailed sleep and heart data inside a confined, operational setting over several months.
The limits of a six-person study
Six volunteers provide only an early view of how crews may respond. A single unusual reading can have a large effect when the group is this small. Statistical tests can identify strong changes, although subtle effects may remain hidden.
The participants were also a selected and experienced group. Several had already taken part in isolation research and one had lived aboard the International Space Station. Their familiarity with confined environments may have reduced the strain caused by alarms, equipment use and the daily mission routine.
Each disturbed condition occurred on a single night, so the experiment could not show what happens when broken sleep continues for many days. Repeated interruptions may gradually reduce performance or alter autonomic recovery even when one disrupted night produces a limited response.
Mission timing may have influenced the measurements as well. Pulse rates and HRV ratios were somewhat higher before isolation and after the crew left the sealed facility. Training demands before entry and readjustment after exit may have raised physical activation. A highly predictable routine during the central part of the mission may have supported stable sleep and recovery.
The study also took place on Earth. Real astronauts experience microgravity, changing light exposure, spacecraft noise and other conditions that can affect the heart and sleep. Future research will need larger groups and repeated sleep disturbances, followed by measurements during actual spaceflight.
Protecting sleep on long space missions
NASA lists sleep loss and disrupted body clocks among the pressures that can add to the health and performance risks of isolation and confinement. Deep-space crews may face communication delays and limited opportunities for outside help, so tired astronauts could need to make difficult decisions on their own.
The SIRIUS-19 results suggest that occasional short awakenings may leave average overnight autonomic measurements fairly stable in healthy, trained people. Sleep quality still declined and the small study cannot rule out weaker effects or damage from repeated disruptions.
A full night awake produced a clearer warning. Pulse rate increased, the HRV balance shifted and most crew members reported greater morning sleepiness. Mission planners can use such evidence when arranging overnight cargo work, emergency drills and recovery periods after unusually long shifts.
Protecting restorative sleep continuity may require quiet sleeping areas and carefully timed lighting. Work schedules can also allow recovery after emergency operations. Portable recorders could alert medical teams when a crew member’s sleep begins to shorten or the body remains highly activated overnight.
Long missions to the Moon and Mars will place people far from normal medical support. Continuous information about sleep and nervous system recovery could help crews manage fatigue before it affects judgment. The four-month experiment indicates that the body can tolerate limited sleep interruption under controlled conditions, while sustained sleep loss creates a stronger and more immediate strain.
