Why Humans Sleep and What Happens Inside the Brain and Body at Night

Scientists still debate the single evolutionary reason humans sleep roughly a third of their lives, but decades of research have mapped in detail what the brain and body actually do while asleep. Two processes dominate: the brain cycles through distinct electrical stages, and across the night it performs maintenance work that is difficult to carry out while awake.

The night is built from repeating 90-minute cycles

Sleep alternates between non-REM (NREM) sleep and REM (rapid eye movement) sleep, according to the U.S. National Institute of Child Health and Human Development (NICHD). NREM itself has three stages: stage 1 is the brief transition into sleep; stage 2 brings slower brain waves and a drop in body temperature; stage 3 is deep sleep, marked by the slowest heart rate and breathing and the release of growth hormone for tissue repair.

REM sleep begins about 90 minutes after falling asleep. The eyes move rapidly, breathing becomes fast and irregular, most muscles are temporarily paralyzed, and the brain becomes highly active, a state the NICHD links to learning and memory consolidation. A full NREM-REM cycle lasts roughly 90 minutes and repeats several times a night, according to the Sleep Foundation, a nonprofit health-information organization. Deep sleep dominates the early cycles; as the night goes on, deep sleep shortens while REM periods lengthen, so that by the final cycles a sleeper spends nearly all their time in stage 2 and REM, per the NICHD.

Across a full night, NREM sleep makes up 75 to 80 percent of total sleep time, with stage 2 alone accounting for roughly half, the Sleep Foundation reports. REM sleep accounts for the remainder.

The brain shrinks slightly to clean itself

One of the clearest functional discoveries came from a 2013 study in the journal Science led by neuroscientist Maiken Nedergaard's team at the University of Rochester. The researchers identified what they named the glymphatic system, a network that uses cerebrospinal fluid flowing through channels around blood vessels to clear metabolic waste from brain tissue, according to a University of Rochester Medical Center (URMC) account of the work.

The team found that brain cells shrink by about 60 percent during sleep, opening more space between cells for fluid to move through, and that the system was almost ten times more active during sleep than during wakefulness, URMC reported. In the same study, sleeping brains cleared more amyloid-beta, a protein associated with Alzheimer's disease, than waking brains did. The neurotransmitter noradrenaline appears to help trigger the cell-size change, according to the same account, though researchers note the full regulatory picture is still being worked out.

Two separate clocks decide when you feel sleepy

Sleep timing is governed by two interacting systems, according to BrainFacts.org, a public-education resource from neuroscience societies including the Society for Neuroscience. The first is the circadian clock, run by the suprachiasmatic nucleus (SCN), a cluster of nerve cells in the hypothalamus. The SCN's cells carry 'clock proteins' that cycle roughly every 24 hours and receive light signals directly from the retina, which is why the clock can be reset by exposure to daylight or by crossing time zones.

The second system is homeostatic sleep pressure, driven by adenosine, a chemical that builds up in the brain the longer a person stays awake. When adenosine binds to receptors on arousal-related brain cells, it dampens their activity and increases drowsiness; sleep clears the buildup. Caffeine works by blocking those same adenosine receptors, which is why it delays the feeling of sleepiness without eliminating the underlying sleep debt, per BrainFacts.org.

How much sleep is recommended, and what happens without it

The U.S. Centers for Disease Control and Prevention (CDC) publishes age-based sleep duration guidance, reproduced below.

The CDC states that consistently getting less than the recommended amount is associated with higher risk of type 2 diabetes, heart disease, high blood pressure, and stroke, and that adequate sleep is linked to fewer illnesses, better mood regulation, and a lower risk of motor-vehicle crashes.

What is still unknown

The glymphatic-clearance findings come from mouse studies; URMC's account notes the mechanism is considered a plausible explanation for part of sleep's restorative function in humans, and that confirming its exact role in human neurodegenerative disease remains an active area of research. Similarly, agencies including NICHD describe memory consolidation and tissue repair as established associations of specific sleep stages rather than a single proven 'purpose' of sleep as a whole.