The International Space Station moves through low Earth orbit at roughly twenty-eight thousand kilometres per hour, about four hundred kilometres above the surface of the planet. At that speed and altitude, one complete circuit takes approximately ninety-two minutes. Which means that as the station moves along its orbital track, the boundary between the sunlit half of Earth and the dark half sweeps across the crew’s field of view sixteen times every twenty-four hours. Sixteen sunrises. Sixteen sunsets. Sixteen periods of forty-five minutes of daylight followed by forty-five minutes of night.
An astronaut on a six-month ISS mission will, over the course of the deployment, watch approximately three thousand dawns and three thousand dusks.
The human body has no biological system designed to accommodate that.
What the body was built for
Almost every biological process in a human being is regulated by an internal timekeeping system that has been calibrated, across roughly the last three billion years of terrestrial evolution, to the specific rhythm of one twenty-four-hour rotation of the planet. It is the oldest continuously maintained biological pattern in every complex organism on Earth. Bacteria have circadian rhythms. Plants have them. Fungi have them. Vertebrate mammals have exceptionally sophisticated versions, integrated into hormone production, body temperature, blood pressure, immune function, cognitive performance, digestion, and the release of every major neurotransmitter the brain uses to regulate mood and attention.
The whole system is entrained by light. Specifically, by the appearance and disappearance of the sun once per day, which the human body treats as the master signal for all other timekeeping decisions. A specialised class of cells in the back of the eye, called intrinsically photosensitive retinal ganglion cells, respond most strongly to blue wavelengths of the kind produced by daylight. When those cells detect blue light in the morning, they send a signal to a small structure in the hypothalamus called the suprachiasmatic nucleus, which resets the body’s master clock and initiates the cascade of hormonal changes that pull a person into wakefulness. When those same cells register the fall of light in the evening, the clock releases melatonin, and the whole system winds down toward sleep.
The system is unusually rigid. Airline pilots crossing time zones experience jet lag because their circadian clock is not designed to shift by more than about an hour a day. Shift workers who alternate between day and night duty have measurably elevated rates of cardiovascular disease, gastrointestinal disorders, cancer, and mood disturbance, on the accumulated evidence of forty years of occupational-health research. The internal timekeeping system is that important, and it is that difficult to override.
Now put a human being on a spacecraft that generates sixteen sunrises a day.
What happens next was documented in careful detail by a 2016 study published in Nature’s npj Microgravity by Dr Erin Flynn-Evans of NASA’s Ames Research Center, together with colleagues from Harvard Medical School and Brigham and Women’s Hospital. The study tracked twenty-one astronauts across a total of 3,248 nights of long-duration ISS spaceflight, using wrist-worn actigraphy devices that recorded sleep-wake patterns in one-minute intervals, alongside photometric measurements of the light environment each astronaut was actually experiencing. The results were unambiguous. Astronauts slept an average of six hours and five minutes per night in space, compared with almost seven hours per night after their return to Earth. On 43.8 per cent of nights aboard the ISS, they slept less than six hours. Sleep-promoting medication use was widespread, with the majority of astronauts taking pharmaceutical sleep aids on a substantial fraction of their in-orbit nights.
The problem was not just the shortness of sleep. It was the misalignment. The astronauts’ internal circadian clocks were drifting out of sync with their imposed twenty-four-hour work schedule at a rate that no ordinary Earth-based intervention could fully correct. Their body temperature rhythms, their hormone release patterns, and their subjective alertness were all landing at the wrong times relative to when their duty roster required them to be awake or asleep. And on the follow-up analysis, the misalignment was associated with measurable decrements in cognitive performance during the specific hours when the astronauts were expected to be operating the most sensitive equipment on the station.
Watch below – Every astronaut who has ever left the planet has come home with a measurably different face:
What NASA is trying to do about it
The engineering solution the agency arrived at, over about a decade of research and testing, was to change the lights.
According to NASA reporting on the installation of the Solid-State Lighting Assembly aboard the International Space Station, completed between 2016 and 2017, the original fluorescent light fixtures aboard the station were replaced with LED panels capable of shifting their colour temperature and intensity across the working day. The new panels have three main operating modes. A general daytime mode that produces balanced white light for ordinary work. An alerting mode that shifts the spectrum toward blue-enriched wavelengths, deployed in the morning and after any period of low alertness, to actively push the astronauts’ circadian clock toward wakefulness. And a pre-sleep mode that removes blue wavelengths entirely, shifting the spectrum toward warm amber tones, which allows melatonin production to begin unimpeded in the two to three hours before the crew is scheduled to sleep.
The lights, on this design, are being used as pharmacological instruments. Not to illuminate the space. To manipulate the crew’s endocrine system into producing the correct hormones at the correct times of day, in a physical environment where the ordinary environmental cues that would normally do that job have been shattered by the orbital mechanics of the vehicle.
Windows on the station, meanwhile, are covered during the crew’s scheduled sleep period, so that none of the sixteen actual dawns and dusks the vehicle is passing through per day can reach the astronauts’ eyes and interfere with the artificially maintained twenty-four-hour cycle. The most photographed views in modern spaceflight, of Earth from orbit at sunrise, are being deliberately hidden from the people who could most easily see them, because seeing them would break the biological system that keeps them functional.
What all of this reveals, on the accumulated NASA operational record, is a specific piece of scientific news that has not entirely reached the public understanding of long-duration spaceflight. The human body is not, on the current biological evidence, particularly well suited to leaving the surface of the planet it evolved on. Almost every physiological system a modern astronaut relies on to function, from bone density to blood pressure to fluid balance to cognitive performance, is being maintained during flight through a combination of engineered countermeasures and pharmaceutical intervention that would not be necessary if the person were still on the ground.
The sleep and circadian problem is, in some ways, the most instructive of these. It cannot be solved by resistance training or by adjusting fluid intake. It has to be solved by tricking the eyes into believing something the surrounding environment is actively contradicting sixteen times a day. And even with the best lighting engineering NASA has been able to design, the astronauts still sleep less than they would on Earth, still take more medication to do so, and still show measurable circadian misalignment throughout their missions.
The three thousand sunrises the average long-duration crew member watches, over the course of a six-month deployment, are one of the more genuinely alien features of the modern human experience. They are also, on the biological evidence, one of the specific reasons the body of an astronaut is quietly under strain in ways that no amount of engineering has yet fully resolved.
Kiran Athar is not a physiologist or a sleep scientist. She writes about biology, human performance, and the ordinary corners of life where the two intersect, drawing on peer-reviewed research and primary-source scholarship.
