Sleep is foundational to nearly every system in the body. During NREM slow-wave sleep, the brain's glymphatic system becomes highly active, with cerebrospinal fluid flow increasing to clear metabolic waste products, including amyloid-beta, a protein implicated in Alzheimer's disease, which is one reason chronic sleep deprivation may raise dementia risk over time. Both too little and too much sleep are associated with higher mortality, with 7 to 8 hours per night representing the typical sweet spot for most adults.
Sleep is essential for memory consolidation, the process by which newly acquired information is stabilized and stored. Declarative memory, the recall of facts and events, depends especially on slow-wave NREM sleep, while procedural memory, such as motor skills, is supported by REM and sleep generally. Beyond memory, sleep supports immune function: deprivation weakens immunity, while adequate sleep strengthens the body's defenses.
Sleep also shapes metabolism and cardiovascular health. Insufficient sleep increases insulin resistance and alters hunger hormones, with leptin, the satiety signal, decreasing and ghrelin, the hunger signal, increasing, which together can drive weight gain. Cardiovascular risk rises with chronic sleep loss due to elevated blood pressure and other mechanisms. Mental health is bidirectionally linked with sleep: depression and anxiety both disrupt sleep and are worsened by it, while insomnia is a recognized risk factor for depression, and trauma-related conditions such as PTSD often produce sleep disruption and recurrent nightmares. Dreams, generated primarily during REM, remain debated in their function, with theories ranging from emotional regulation and memory processing to threat simulation and neural maintenance, as in the activation-synthesis hypothesis, in which dreams are the brain making sense of random neural activity.