Living and Working in Space: Health, Daily Life, and the Human Body in Orbit

The International Space Station orbits Earth at roughly 28,000 kilometers per hour, completing a full orbit every 90 minutes. Its crew — typically six or seven people — experiences 16 sunrises and sunsets every day, floats freely through the modules, and must contend with an environment that the human body was never designed to inhabit. Understanding how astronauts live and work in this extreme environment illuminates not just the practical challenges of spaceflight but the physiological frontiers that stand between us and permanent human presence beyond Earth.

What Microgravity Does to the Human Body

The moment a spacecraft reaches orbit and engines cut off, everything that gravity has been doing to the human body — compressing the spine, pooling blood in the legs, prompting bones and muscles to constantly work against their own weight — stops. The consequences begin immediately and are profound.

Fluid shift and the puffy face. On Earth, gravity pulls body fluids downward, concentrating blood and other fluids in the legs and lower body. In microgravity, this gradient disappears and fluids redistribute evenly, moving toward the head and upper body. Within hours of reaching orbit, astronauts develop nasal congestion, a puffy face, and what crews describe as the sensation of being permanently slightly upside down. The legs meanwhile look thinner — sometimes called "bird legs" — as fluid that normally pools there redistributes upward. The body compensates by producing hormones that reduce total fluid volume, so astronauts urinate more in the first days of flight.

Bone density loss. Bone is living tissue, constantly being remodeled. On Earth, the mechanical stress of gravity and muscle load on the skeleton stimulates bone formation. In microgravity, that mechanical stimulus vanishes and bone density begins to decrease at a rate of approximately 1 to 2 percent per month in weight-bearing bones like the hips and lower spine. Over a six-month mission, an astronaut can lose 10 to 15 percent of bone density in the hip — equivalent to decades of osteoporosis aging in a matter of months. This is one of the most serious medical challenges for long-duration spaceflight and a major focus of ISS research.

Muscle atrophy. Without gravity to push against, muscles begin to waste surprisingly quickly. The postural muscles — those in the back, legs, and core that spend every moment on Earth maintaining your upright position against gravity — are hit hardest. Within a few weeks of weightlessness, significant muscle loss occurs if countermeasures are not applied. This is why ISS crew members are required to exercise for at least 2.5 hours every single day, a non-negotiable component of their schedule.

Vision changes and VIIP syndrome. One of the most surprising discoveries of the ISS era has been a condition now called Spaceflight-Associated Neuro-ocular Syndrome (SANS), previously known as Visual Impairment and Intracranial Pressure (VIIP) syndrome. The upward fluid shift increases pressure in the skull, which flattens the back of the eyeball (the globe), shifts the optic nerve, and causes farsightedness. Studies show that more than half of male astronauts on long-duration missions develop measurable vision changes, some of which are permanent. Female astronauts appear less affected, possibly due to hormonal differences. SANS is considered one of the key health risks for any human mission to Mars.

Radiation exposure. The ISS orbit is above much of Earth's protective magnetosphere, exposing crew to radiation levels roughly 100 times higher than on Earth's surface. Over a six-month mission, an astronaut accumulates roughly 80 millisieverts of radiation — equivalent to receiving about 800 chest X-rays. This increases lifetime cancer risk and damages DNA. On a mission to Mars, which would involve leaving Earth's magnetosphere entirely, radiation exposure would be far greater and is a significant unsolved problem for deep-space human exploration.

Immune system changes. Research has shown that the immune system behaves abnormally in space. Immune cells function less effectively, and latent viruses — including herpesviruses that lie dormant in most adults — can reactivate. This has implications both for crew health during missions and for understanding immune function more broadly, with potential applications for aging research and immunology on Earth.

A Day in the Life: The ISS Schedule

Life aboard the ISS is highly structured. With crew members from multiple nations, multiple mission control centers on the ground, and a packed schedule of experiments, maintenance, and communications, the day follows a carefully managed routine.

Wake-up: approximately 6:00 AM GMT. The day begins with personal hygiene — a process far more complicated than it sounds without running water — followed by breakfast. Each morning starts with a daily planning conference, a video call with all relevant mission control centers: Houston for NASA, Moscow for Roscosmos, Cologne for ESA, Tsukuba for JAXA, and Montreal for CSA. This conference covers the day's work schedule, any anomalies overnight, and crew health updates.

Work period: 8:00 AM to 1:00 PM and 2:30 PM to 5:30 PM. The working day runs approximately 10 hours, divided between morning and afternoon sessions with lunch in between. Crew time is divided among scientific experiments (maintaining ongoing research racks, performing new experiments, collecting biological samples), station maintenance (inspections, repairs, software updates, hardware swaps), and preparation for upcoming events like visiting vehicle dockings or spacewalks.

Exercise: 2.5 hours mandatory. Sandwiched into the schedule every day is the most critical health activity: exercise. The ISS houses three exercise devices specifically designed for the microgravity environment. The Advanced Resistive Exercise Device (ARED) uses vacuum cylinders to simulate up to 270 kilograms of free-weight resistance, allowing astronauts to perform squats, deadlifts, and upper-body exercises that load the bones and preserve muscle mass. The T2 treadmill has crew members running while attached by bungee cords that apply a downward force to prevent them from floating off the surface. The CEVIS (Cycle Ergometer with Vibration Isolation and Stabilization) is a stationary bike system. Without this daily regimen, bone density and muscle mass would decline to the point where returning crew members could not walk unassisted.

Free time and sleep: 8:30 PM lights-out. The crew has roughly two to three hours of personal time in the evenings before an 8.5-hour sleep period. The ISS has no natural day-night cycle — the sun rises every 90 minutes — so the station's lighting systems simulate a normal day-night rhythm using the same principles as circadian lighting research. The sleep period targets 6:00 AM GMT wake-up.

Food in Space

Eating aboard the ISS requires more creativity than most people imagine. The constraints are real: no crumbs (they float into eyes, noses, and air systems), no open liquids, limited refrigeration, and the fact that fluid shifts change taste perception — astronauts often report food tasting blander in space because nasal congestion dulls smell, which accounts for much of our sense of taste.

No bread. Standard sliced bread is banned because crumbs are a genuine hazard in a microgravity environment — they float freely and can get into lungs, equipment, and eyes. Tortillas are the standard substitute. NASA worked with a tortilla manufacturer to extend their shelf life so they could last for weeks or months without refrigeration.

Food forms. ISS food comes in several forms. Thermostabilized pouches (essentially retort pouch meals, similar to military MREs) are the most common. Freeze-dried foods are rehydrated with hot or cold water from a gun-style dispenser. Irradiated meats can last at room temperature for months. Condiments like salt and pepper must be in liquid suspension — dry powders would float away and create the same crumb problem as bread.

Fresh food. The most prized food on the ISS is fresh produce delivered by cargo vehicles. Apples, oranges, fresh vegetables — foods that simply cannot be long-shelf-stored — arrive on supply missions and are consumed as quickly as possible. Since 2015, the ISS has also grown small quantities of lettuce, radishes, and peppers in the Veggie plant growth facility, and crew members have eaten space-grown produce for the first time in history. Space farming remains a research priority for long-duration missions where resupply is not possible.

No alcohol. Alcohol is officially prohibited aboard the ISS, primarily because it interferes with the body's adaptation to microgravity and could compromise the judgment required for safety-critical operations. Russian crews historically had more flexibility in this regard, but current ISS policies for all segments are dry.

Hygiene Without Running Water

Personal hygiene in microgravity requires entirely different products and techniques from anything used on Earth. Water behaves strangely in weightlessness, forming floating globules that can be inhaled, get into electronics, or simply escape.

Washing up. Crew members use rinseless shampoo — formulated without the need for water rinse — to wash their hair. They apply it, work it through with a towel, and the job is done. No-rinse body wash products similarly allow sponge-bath-style cleaning without the need to rinse. For oral hygiene, astronauts brush their teeth normally but swallow the toothpaste rather than spitting, since free-floating spit presents the same hazards as other liquids.

The toilet. The ISS toilet (technically the Waste Collection System) uses airflow and suction rather than water and gravity to direct waste. Liquid waste is processed through the Water Recovery System, which purifies urine back to drinking water with a purity that exceeds most municipal tap water supplies. Solid waste is collected, compacted, sealed in bags, and stored until it can be loaded onto a disposal cargo vehicle to burn up during reentry. The ISS recycles approximately 93 percent of all water aboard, including urine and humidity from exhaled breath — a technology that will be essential for long-duration deep-space missions.

Sleep in Orbit

Sleeping in space presents unique challenges. There is no up or down, so crew members sleep in sleeping bags attached to the wall or ceiling of their small private crew cabins. The bags have arm holes so that arms do not float out freely during sleep, which would be disorienting. Without the bag, a sleeping astronaut's arms drift forward into a relaxed floating position in front of them.

The ISS is loud. Life support fans, pumps, and equipment run continuously, producing a constant low-level noise equivalent to a quiet office. Most crew members use earplugs to block the sound. The 90-minute orbital period means that sunlight appears and disappears rapidly if sleeping near a window, so eye masks are standard. The station's lighting system has been upgraded to LED fixtures that can simulate natural light cycles, improving circadian rhythm maintenance.

Sleep quality in space tends to be worse than on Earth, particularly in the first weeks of a mission as the body adjusts. Studies have found that ISS crew members average about 6 hours of sleep per night despite an 8.5-hour sleep period, and that melatonin and other sleep aids are commonly used during missions.

Communications and Psychological Well-Being

The psychological dimension of long-duration spaceflight is as important as the physiological. Crews live and work in a confined space roughly equivalent to a five-bedroom house, with the same small group of colleagues, for six months or more. Earth is 400 kilometers below and completely inaccessible.

Communications. Astronauts on the ISS have significant communications access with the ground. Email is available continuously. Video calls home are scheduled regularly. Social media access is permitted — many ISS astronauts maintain active accounts and have posted photographs and videos that have brought the experience of spaceflight to millions of people. The communications delay to the ISS is milliseconds, essentially real-time, which makes these connections feel natural. On a mission to Mars, the communications delay would be 4 to 24 minutes each way, fundamentally changing the nature of connection with Earth.

Psychological challenges. Isolation, confinement, the monotony of a highly structured schedule, interpersonal friction in close quarters, and the constant awareness of danger create real psychological stress. NASA and other space agencies have developed extensive psychological support programs: weekly private consultations with psychologists, family communication opportunities, care packages delivered on cargo vehicles, and even attempts to replicate seasonal events and holidays. A sense of humor appears to be one of the most reliable coping mechanisms reported by long-duration crew members.

Extended missions and the Crew-11 precedent. The SpaceX Crew-11 mission, launched in 2024, saw astronauts Butch Wilmore and Suni Williams remain aboard the ISS for significantly longer than their planned eight-day stay when technical issues with their Boeing Starliner vehicle grounded the return spacecraft. They ultimately returned on a SpaceX Crew Dragon after spending approximately nine months on the station. Their extended stay provided unexpected data on long-duration adaptation and highlighted the psychological resilience required of modern astronauts, who must adapt gracefully to radical changes in their expected mission timeline.

Coming Home: Readaptation

Returning to Earth after a long-duration mission is its own major physiological challenge. After six months in microgravity, the body has comprehensively adapted to the absence of gravity, and suddenly being subjected to 1g again is profoundly disorienting.

Upon landing, astronauts are typically carried from their Soyuz or Crew Dragon capsule and cannot stand or walk unassisted. The vestibular system, which governs balance and spatial orientation, has recalibrated for weightlessness and takes days to weeks to readjust. The cardiovascular system, which no longer has to pump blood upward against gravity, has reduced its efficiency and struggles initially with orthostatic hypotension — a drop in blood pressure when standing upright. Crew members often experience dizziness and near-fainting in the first hours and days after landing.

Muscle strength begins recovering within days. Bone density recovery is slower and may take months to years to fully return, and some studies suggest that certain bone changes may be permanent after very long missions. Fluid balance normalizes within days. The puffy face and bird legs resolve quickly. Vision changes can persist or be permanent in severe cases of SANS.

Full readaptation typically takes several weeks to a few months, with crew members following a structured rehabilitation program of physical therapy, cardiovascular exercise, and balance training. They are not permitted to drive a vehicle for several weeks after landing.

Record Holders and the Long View

The record for total cumulative time in space belongs to Russian cosmonaut Oleg Kononenko, who surpassed 878 days in space across five missions by early 2024 — more than any human in history. His record, and those of others who have spent extended time on the ISS, have provided invaluable data on the body's long-term adaptation to spaceflight.

The longest single spaceflight by an American is held by Mark Vande Hei, who spent 355 consecutive days aboard the ISS in 2021-2022. The overall single-mission record is held by Soviet cosmonaut Valeri Polyakov, who spent 437 days continuously aboard the Mir space station in 1994-1995 — a record that stood for decades as the definitive test of human endurance in orbit.

These long-duration missions are not merely feats of endurance. They are essential preparation for the next frontier: a crewed mission to Mars, which would require approximately six to nine months in transit each way, plus surface time, for a total mission duration of two to three years. Everything we learn about the human body in space — every countermeasure developed, every syndrome identified and mitigated — brings that mission one step closer to the realm of the achievable. The ISS is not just a research laboratory. It is humanity's training ground for deep space.