Sleep

Chronotype is the individual disposition toward earlier or later sleep-wake timing, commonly described as morning, intermediate, or evening type. It is shaped by genetics, age, light exposure, and social schedules. Chronotype influences cognitive peak times, athletic performance, and cardiometabolic risk, and a mismatch with imposed work or school hours, known as social jetlag, has been linked to obesity, mood disorders, and impaired metabolic health.

The circadian rhythm is the body's roughly 24-hour internal cycle that coordinates sleep-wake timing, hormone release, body temperature, and metabolism. It is governed by the suprachiasmatic nucleus in the hypothalamus and entrained primarily by light exposure. Stable circadian alignment supports cardiometabolic health, immune function, and cognitive performance, while chronic disruption is linked to obesity, type 2 diabetes, and accelerated biological aging.

The cortisol awakening response (CAR) is a sharp rise in salivary cortisol of roughly 50 percent on average (commonly reported in the range of about 38 to 75 percent) from the awakening sample to a peak about 30 to 45 minutes after waking. It reflects healthy hypothalamic-pituitary-adrenal axis activation, mobilising energy and focus for the day. A blunted or exaggerated CAR is associated with chronic stress, burnout, depression, sleep disorders, and adverse cardiometabolic outcomes, making it a useful marker in longevity and stress research.

Deep sleep, or slow-wave sleep (N3), is the stage characterised by high-amplitude delta waves on EEG and the highest arousal threshold. It dominates the first third of the night and drives growth hormone release, cardiovascular recovery, immune regulation, and glymphatic clearance of metabolic waste. Deep sleep declines with age, and lower amounts are associated with poorer memory and increased risk of neurodegenerative disease.

The glymphatic system, described by Iliff, Nedergaard and colleagues in 2012, is the brain's waste-clearance pathway, in which cerebrospinal fluid flows along perivascular spaces, exchanges with interstitial fluid, and removes metabolic byproducts such as beta-amyloid and tau. Activity increases substantially during deep sleep, when the interstitial space expands by roughly 60 percent (Xie et al., 2013). Impaired glymphatic clearance is implicated in Alzheimer's disease and other neurodegenerative conditions, making sleep a key intervention point for brain longevity.

Melatonin is a hormone secreted by the pineal gland in response to darkness, signalling biological night and helping align the circadian system. It facilitates sleep onset, modulates core body temperature, and exerts antioxidant effects. Endogenous melatonin declines with age, and bright evening light suppresses its release. Low-dose exogenous melatonin is used to address jet lag, shift work, and delayed sleep-phase patterns.

REM (rapid eye movement) sleep is a sleep stage marked by fast eye movements, vivid dreaming, near-waking brain activity, and skeletal muscle atonia. It increases toward the second half of the night and supports memory consolidation, emotional processing, and synaptic plasticity. Reduced REM duration has been associated in epidemiological studies with higher all-cause mortality, cognitive decline, and impaired mood regulation.

Sleep apnea is a disorder of repeated breathing pauses or shallow breathing events (apneas and hypopneas) during sleep, most commonly obstructive sleep apnea from upper-airway collapse, less often central sleep apnea from disrupted respiratory drive. The AASM diagnoses it at an apnea-hypopnea index of at least 5 per hour with symptoms, or at least 15 without. Untreated, it raises the risk of hypertension, atrial fibrillation, stroke, type 2 diabetes, cognitive decline, and all-cause mortality.

Sleep efficiency is the percentage of time spent asleep relative to the total time in bed, calculated as total sleep time divided by time in bed. Values of 85 percent or higher are generally regarded as healthy in adults. Low sleep efficiency reflects fragmented or inefficient sleep and is associated with daytime fatigue, impaired glucose metabolism, elevated cardiovascular risk, and poorer subjective quality of life.

Sleep latency is the time from lights-out to the first epoch of sleep, typically measured in minutes during polysomnography. A latency of about 10 to 20 minutes is considered healthy; very short latencies (under roughly 5 to 8 minutes) can indicate sleep deprivation or excessive daytime sleepiness, while persistently longer values suggest insomnia or circadian misalignment. It is a core metric in polysomnography and consumer sleep trackers used in longevity contexts.