Why do we sleep and what the research measures

The question of why do we sleep remains one of the central inquiries in human biology. Despite spending roughly one third of human life in this state, scientific investigation has not produced a single consensus answer. Instead, investigators evaluate several distinct mechanisms across metabolism, cognition, and cellular maintenance. This article describes what research has measured and proposed about why people sleep. It gives no advice on how long or when to sleep.

Current scientific understanding of sleep functions

A 2005 review of sleep in mammals stated that the functions of sleep remain unclear. It noted that most theories give non-REM sleep a role in energy conservation and in recuperation of the nervous system, and it proposed that sleep may be an efficient time for the completion of a number of functions, while noting that variation in how sleep is expressed across mammals indicates those functions may differ between species ^[1]. Rather than a single master purpose, current inquiries examine multiple physiological processes that occur simultaneously or sequentially during resting states.

Energy expenditure during sleep and wakefulness

One long-standing hypothesis proposes that sleep serves primarily to conserve metabolic energy. In a laboratory study, seven healthy adults with a mean age of 22 lived in a whole-room calorimeter through a baseline day with 8 hours of scheduled sleep, 40 hours without sleep and 8 hours of recovery sleep. Compared with baseline, energy expenditure over 24 hours rose by about 7 percent during the first 24 hours without sleep, and energy expenditure at night rose by about 32 percent on the night without sleep ^[2].

CIBDOL · Energy expenditure during sleep and wakefulness

In the same calorimeter study, energy expenditure during the recovery night was about 4 percent lower than during the baseline night. Differences between sleep stages were small, while time awake during the sleep episode was associated with higher energy expenditure, and the authors wrote that their findings support the hypothesis that sleep conserves energy ^[2].

Memory retention and different sleep intervals

Another prominent line of investigation concerns how sleep interacts with neural information storage. A 2013 review of sleep and memory research states that more than a century of research has established that sleep benefits the retention of memory. It describes current theories in which memories are actively consolidated during sleep, through the reactivation of recently encoded memory representations during slow wave sleep, with the REM sleep that follows possibly stabilising them ^[3].

The relationship between specific sleep stages and memory types was evaluated experimentally. In a 1997 experiment with 20 healthy men, sleep generally enhanced recall compared with a matching retention interval of wakefulness. Recall of word pairs improved more across early night sleep, which contained five times as much slow wave sleep as the late interval, and a mirror-tracing skill improved more across late night sleep, which contained twice as much REM sleep as the early interval ^[4].

Synaptic homeostasis and brain plasticity

Beyond sorting specific memories, researchers also evaluate how sleep influences overall neural connectivity. The synaptic homeostasis hypothesis, set out by its authors in a 2014 perspective, proposes that sleep is the price the brain pays for plasticity. According to the hypothesis, learning while awake requires strengthening connections throughout the brain, which increases cellular needs for energy and supplies, and during sleep spontaneous activity renormalises net synaptic strength. The perspective considers the rationale and evidence for the hypothesis and points to open issues related to sleep and plasticity ^[5].

Direct evidence for regional changes during rest has been documented in people. A 2004 study of human sleep found that a learning task involving specific brain regions was followed by a local increase in slow wave activity during the following sleep, and that the local increase correlated with better performance on the task after sleep ^[6].

Brain fluid dynamics during rest

A further hypothesis addresses the physical clearance of metabolic by-products. In a 2013 study in mice, natural sleep and anaesthesia were both associated with a 60 percent increase in the space between brain cells, an increase in the exchange of cerebrospinal fluid with the fluid between cells, and a faster clearance of beta-amyloid. This is an animal finding. The authors proposed that the restorative function of sleep may be a consequence of the removal of waste products that accumulate in the awake central nervous system ^[7].

Translating these observations to human biology requires careful measurement. A 2019 neuroimaging study in people recorded a coherent pattern during non-REM sleep in which neural slow waves were followed by haemodynamic oscillations, which were in turn coupled to waves of cerebrospinal fluid flow in the brain. The study measured brain rhythms, haemodynamics and fluid flow; it did not measure the clearance of any waste substance ^[8]. Together, these experimental lines demonstrate that sleep encompasses a suite of coordinated biological processes rather than one isolated function.