Sleep: Stages, Cycles and Influences
How Do You Sleep? The Stages and the Cycle
Sleep is not one single state. Across a night, your brain moves through a repeating pattern of four sleep stages — light sleep, deeper sleep, deep (slow-wave) sleep, and REM sleep. One complete pass through all four stages is called a sleep cycle. During REM, the eyes move rapidly beneath closed lids, brain activity looks similar to being awake, and most vivid dreaming happens — yet the major muscles are temporarily paralysed so the sleeper cannot act out what they dream. Heart rate and breathing also become faster and more irregular during REM, closer to waking levels than to the calm, slow rhythms of deep sleep.
A full sleep cycle lasts about 90 minutes and repeats four to five times a night. Early cycles contain more deep sleep; later cycles, closer to morning, contain more REM sleep.
Why the Cycle Matters
Each stage does a different job. Deep, slow-wave sleep restores the body physically — tissue repairs, the immune system is strengthened, and growth hormone is released. REM sleep supports memory consolidation and emotional processing, helping the brain sort and store the day’s experiences. Because the cycle repeats several times each night, the body gets repeated doses of both types of restoration rather than just one long dose of either.
This is also why the balance shifts across the night. The body prioritises physical repair early on, when deep sleep dominates, and shifts towards memory and emotional processing later, when REM dominates. If a person is woken early or their sleep is cut short, they lose a disproportionate amount of REM sleep, since it is concentrated in the later cycles — this is one reason chronic short sleep is linked to poorer memory and mood.
Light sleep is the transition out of wakefulness — breathing and heart rate begin to slow and the body relaxes. Deeper sleep relaxes the body further, lowering body temperature and slowing brain activity. Deep, slow-wave sleep is the hardest stage to wake from; it is when growth hormone is released and physical repair happens. REM then follows, where brain waves resemble the waking brain and dreaming is at its most vivid.
Waking suddenly out of REM often leaves a person feeling groggy and disoriented, because the brain has to switch quickly from a highly active dreaming state back to full alertness. This is sometimes called sleep inertia, and it is generally most intense when a person is woken abruptly out of deep, slow-wave sleep.
Internal Influences: Bodily Rhythms
A circadian rhythm is a roughly 24-hour biological cycle controlled by the body’s internal clock. The sleep–wake pattern is the clearest example — the drive to sleep and wake follows an approximate 24-hour pattern even without any clocks or daylight to check.
An ultradian rhythm is a biological cycle that repeats several times within a single day. The sleep cycle itself is an ultradian rhythm, since it repeats roughly every 90 minutes throughout the night rather than once every 24 hours like a circadian rhythm does.
Circadian rhythms repeat roughly every 24 hours, like the sleep–wake cycle. Ultradian rhythms repeat several times within a single day, like the 90-minute sleep cycle.
Hormones: The Pineal Gland and Melatonin
The pineal gland is a small gland in the brain that produces melatonin. As daylight fades in the evening, the eyes detect the drop in light and signal the pineal gland to release more melatonin, which makes a person feel sleepy. As light increases again in the morning, melatonin production falls, helping the body feel alert and ready for the day.
External Influences: Zeitgebers
A zeitgeber is an external cue that helps keep the body’s internal clock synchronised to the 24-hour day. Light is the most powerful zeitgeber: it signals the brain to suppress melatonin production during the day and allow it to rise at night, resetting the circadian rhythm each day. Without regular exposure to light, the internal clock can drift away from a 24-hour pattern.
This matters in everyday life. Bright artificial light in the evening — including the blue light from phone and laptop screens — can trick the pineal gland into delaying melatonin release, making it harder to fall asleep at the usual time. Shift workers who sleep during the day and stay awake at night often struggle for the same reason: their light exposure is the opposite of what their circadian rhythm expects, so the two fall out of sync.
Michel Siffre spent six months living alone in a cave in Texas, cut off from natural light, clocks and any other cues about the time of day. The aim was to see what happened to his circadian rhythm when all zeitgebers were removed.
Siffre’s sleep–wake cycle drifted away from 24 hours, sometimes stretching towards 48 hours. He also lost track of time, believing far fewer days had passed than actually had. With no zeitgebers to reset it, his body clock began to “free-run” on its own internal schedule instead of following the outside world.
Siffre concluded that zeitgebers, especially light, are needed to keep the circadian rhythm synchronised to a 24-hour day. A strength of the study is that it produced detailed, real data over an unusually long period of isolation that would be very difficult to arrange in a laboratory. A weakness is that it was a case study of just one person, which limits how far the findings can be generalised to other people.