Sleep

Actigraphy uses a wrist-worn accelerometer to infer sleep-wake states from movement patterns over days to weeks, providing an ambulatory and low-burden alternative to polysomnography for longitudinal sleep assessment. Validated algorithms translate raw activity counts into estimates of total sleep time, sleep efficiency, sleep-onset latency, and waking after sleep onset. It is recommended by the American Academy of Sleep Medicine for evaluating insomnia, circadian rhythm disorders, and treatment response in naturalistic settings. Actigraphy systematically overestimates total sleep time and sleep efficiency relative to PSG, particularly in patients with insomnia, and cannot reliably stage sleep.

Adenosine is a purine nucleoside that accumulates in the brain during wakefulness as a byproduct of neuronal energy metabolism and acts on A1 and A2A receptors to promote sleepiness and suppress arousal, serving as the primary molecular mediator of homeostatic sleep pressure. Its levels are highest after prolonged waking and decline during sleep. Caffeine exerts its alerting effects principally by competitively blocking adenosine receptors without depleting adenosine itself, which is why the sleepiness rebound after caffeine clearance is pronounced. Beyond sleep regulation, adenosine is involved in cerebrovascular autoregulation and has been implicated in the glymphatic clearance process that intensifies during slow-wave 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.

Dim Light Melatonin Onset (DLMO) is the time of evening at which endogenous melatonin in saliva or plasma rises above a defined threshold under dim ambient light (typically below 50 lux) which would otherwise suppress secretion. Standard thresholds are around 3 pg/mL in saliva or 10 pg/mL in plasma, sampled at 30-minute intervals. Formalised by Alfred Lewy and colleagues, DLMO is widely regarded as the gold-standard phase marker of the human circadian system because melatonin secretion is tightly controlled by the suprachiasmatic nucleus and is minimally confounded by sleep, posture, or activity. Clinically, DLMO is used to diagnose delayed and advanced sleep-wake phase disorders (DSPS/ASPS), to time exogenous melatonin and light therapy, to manage shift work and jet lag, and as a research outcome in chronobiology trials.

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 sleep (and under anaesthesia in animal models), 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.

Insomnia is a sleep disorder defined by persistent difficulty initiating or maintaining sleep — or non-restorative sleep — despite adequate sleep opportunity, causing clinically meaningful daytime impairment. Per DSM-5 and ICSD-3, chronic insomnia disorder requires symptoms on at least three nights per week for at least three months. Approximately 10% of the adult population meets diagnostic criteria; prevalence rises with age, making it among the most common sleep complaints in older adults. The pathophysiology centers on hyperarousal: sustained activation of the HPA axis and the sympathetic nervous system elevates nighttime cortisol, raises core body temperature, and increases whole-brain glucose metabolism — all antagonizing sleep onset and maintenance. Prospective cohort data raise concern for long-term consequences: a 2019 meta-analysis by Ge et al., pooling 29 cohorts with over 1.5 million participants, found that difficulty initiating sleep and non-restorative sleep were independently associated with elevated all-cause and cardiovascular mortality risk; insomnia disorder as a diagnostic entity and difficulty maintaining sleep did not reach statistical significance. Cognitive behavioral therapy for insomnia (CBT-I) — combining sleep restriction, stimulus control, cognitive restructuring, and sleep-hygiene education — is the first-line treatment per the 2016 American College of Physicians guideline; pharmacotherapy is adjunctive and carries greater risk in older populations.

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.

Orexin-A and orexin-B (also called hypocretin-1 and hypocretin-2) are two excitatory neuropeptides produced by a small population of neurons in the lateral and posterior hypothalamus, independently identified in 1998 by De Lecea and colleagues (as hypocretins, PNAS) and Sakurai and colleagues (as orexins, Cell). They act on two G-protein-coupled receptors (OX1R and OX2R) and project broadly to wake-promoting nuclei including the locus coeruleus, tuberomammillary nucleus, and basal forebrain. The system stabilises wakefulness and gates state transitions between wake, NREM, and REM sleep. Selective loss of orexin neurons - usually autoimmune - causes narcolepsy type 1 with cataplexy. Dual orexin receptor antagonists (DORAs), including suvorexant, lemborexant, and daridorexant, exploit this axis to treat chronic insomnia.

Polysomnography (PSG) is the gold-standard multi-channel sleep study conducted in a laboratory setting, simultaneously recording electroencephalography (EEG), electrooculography (EOG), electromyography (EMG), electrocardiography (ECG), respiratory airflow, effort, and oxygen saturation. Sleep stages—N1, N2, N3 (slow-wave), and REM—are scored in 30-second epochs according to the AASM manual using EEG, EOG, and EMG channels. PSG is the reference standard for diagnosing obstructive and central sleep apnea (via apnea-hypopnea index), narcolepsy, REM sleep behavior disorder, and periodic limb movement disorder. Consumer wearables and actigraphy are validated against PSG but typically underestimate N3 and N1 and overestimate sleep efficiency.

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 architecture refers to the cyclical organisation of sleep stages across the night, typically comprising four to six 90-minute ultradian cycles each progressing through N1, N2, N3 (slow-wave sleep), and REM, with N3 dominating early cycles and REM expanding in later ones. Healthy young- to middle-aged adult sleep contains roughly 13–23% N3 and 20–25% REM (with N3 declining to <10% by age 70), though normative ranges vary with age, sex, and assessment method. Disruptions to architecture—including suppression of N3 by alcohol, fragmentation of REM by sleep apnea, or the age-related decline of slow-wave sleep—have functional consequences for memory consolidation, hormonal secretion, immune regulation, and cardiovascular recovery, making architectural metrics a key target in longevity-oriented sleep assessment.

Sleep debt is the cumulative shortfall between an individual's biologically required sleep duration and the sleep actually obtained over consecutive nights. It builds incrementally: restricting sleep to 6 h per night for 14 days produces neurobehavioral impairment equivalent to up to two nights (~48 h) of total sleep deprivation, yet affected individuals consistently underestimate their own impairment — a key finding from the controlled dose-response experiment by Van Dongen et al. (2003) in 48 healthy adults. On the metabolic side, even six nights of curtailment to 4 h per night shifts glucose tolerance, elevates evening cortisol, and raises sympathetic nervous system activity toward profiles seen in normal aging — as Spiegel, Leproult, and Van Cauter (Lancet, 1999) demonstrated in 11 healthy young men. Weekend "catch-up" sleep provides partial but incomplete recovery: Åkerstedt et al. (2019), following a Swedish cohort of 43,880 adults for 13 years, found that short weekday sleep paired with long weekend sleep carried no excess mortality in adults under 65, whereas persistently short sleep on both days was associated with a roughly 65 % higher mortality rate (HR 1.65) — suggesting compensatory sleep can buffer some long-term risk but cannot reliably undo all within-week deficits. The causal direction between chronic sleep debt and outcomes such as cardiovascular disease, insulin resistance, and accelerated cognitive aging is supported by controlled human experiments, though the magnitude of irreversible damage from years of mild restriction remains associational in large-scale epidemiology.

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.

The two-process model, proposed by Alexander Borbély in 1982, describes sleep-wake regulation as the interaction of two independent processes: Process S (homeostatic sleep pressure), which accumulates as adenosine and other somnogens build up during waking and dissipates during sleep, and Process C (the circadian signal), generated by the suprachiasmatic nucleus and imposing a roughly 24-hour oscillation in alerting drive that opposes increasing sleep pressure toward evening to maintain sustained wakefulness. Sleep occurs when Process S exceeds the circadian alerting threshold. The model successfully explains phenomena such as the post-lunch dip, rebound deep sleep after sleep deprivation, and the sharp morning wake boundary, and remains the dominant framework for circadian and sleep medicine research.

Sleep regularity is the day-to-day consistency of an individual's sleep-wake timing, quantified by the Sleep Regularity Index (SRI) — a score from −100 to 100 (with 0 representing fully random patterns and 100 representing perfectly identical days) that calculates the average probability of being in the same sleep or wake state at any two time points exactly 24 hours apart across a 7-day actigraphy recording. Unlike sleep duration, which measures how long a person sleeps, the SRI captures whether sleep occurs at the same clock times each day, reflecting the stability of circadian entrainment. In a prospective cohort study of 60,977 UK Biobank participants followed for 6.3 years (Windred et al., 2024), participants in the highest SRI quintile had 20–48% lower all-cause mortality risk, 16–39% lower cancer mortality, and 22–57% lower cardiometabolic mortality compared with the least regular quintile; when both SRI and sleep duration were entered simultaneously, duration added no independent predictive value. A separate UK Biobank analysis (Cribb et al., 2023, n = 88,975) confirmed a non-linear relationship, with the lowest SRI percentile (score ≈ 41) carrying a 53% higher all-cause mortality hazard relative to the median. The proposed mechanism is that chronic irregularity disrupts circadian timing — misaligning peripheral organ clocks with the central suprachiasmatic pacemaker — thereby promoting inflammation, hypothalamic-pituitary-adrenal axis dysregulation, autonomic dysfunction, and gut dysbiosis; however, both large studies are observational, so causality remains unconfirmed.

Sleep spindles are bursts of rhythmic neural activity in the 11–16 Hz range — visible on EEG as waxing-and-waning sigma-band oscillations that define NREM stage N2 sleep. They originate in the thalamic reticular nucleus (TRN), whose GABAergic neurons hyperpolarize thalamocortical relay cells in rhythmic bursts; oscillations propagate via thalamocortical axons to widespread cortical areas, lasting 0.5–2 seconds. Spindles coordinate hippocampal-to-neocortical memory consolidation: each burst opens a window of heightened cortical excitability time-locked with hippocampal sharp-wave ripples and cortical slow oscillations (~0.75 Hz), enabling targeted memory replay. Spindle density (events per minute) and peak frequency are measured by polysomnography (PSG) or high-density EEG (hdEEG). Fast spindles (13.5–15 Hz, fronto-central) are most consistently linked to declarative and episodic memory; slow spindles (11–13 Hz, parietal) correlate more weakly. With normal aging, fast spindle density declines markedly — by more than 40% over prefrontal regions in older versus younger adults — and Mander et al. (2014, Cerebral Cortex) showed this mediates age-related episodic learning deficits via impaired hippocampal activation. The association between spindle metrics and memory performance is consistent in cross-sectional human studies; causal evidence from interventional trials in healthy older adults remains limited, and spindle-boosting strategies (e.g., zolpidem, acoustic closed-loop stimulation) are still investigational.

Social jetlag is the chronic discrepancy between an individual's endogenous circadian timing and the sleep-wake schedule imposed by social obligations such as work or school, quantified as the absolute difference in midsleep time between free days and workdays. The term was introduced by Wittmann, Dinich, Merrow, and Roenneberg in 2006 and measured in hours using the Munich Chronotype Questionnaire (MCTQ); approximately 69–70% of adults in industrialized countries exhibit at least one hour of misalignment. Late chronotypes are disproportionately affected because social start times are earlier than their biological clocks prefer. Roenneberg et al. (2012, Current Biology) demonstrated in a large European cohort that social jetlag was associated with an odds ratio of 3.3 (95% CI 2.5–4.3) for belonging to the overweight group, independent of sleep duration — one of the first large-scale human associations between circadian misalignment and adiposity. Proposed mechanisms include HPA-axis dysregulation with elevated cortisol, autonomic nervous system activation, appetite-hormone shifts (elevated ghrelin, reduced leptin), impaired insulin sensitivity, and reduced physical activity, all converging on metabolic dysfunction. Evidence for cardiovascular risk remains largely associational in humans; experimental misalignment studies confirm acute physiological changes, but randomized long-term intervention trials demonstrating that correcting social jetlag improves hard cardiometabolic endpoints are still lacking as of 2026.

The suprachiasmatic nucleus (SCN) is a paired hypothalamic structure situated immediately above the optic chiasm, containing approximately 20,000 neurons per side. It functions as the master circadian pacemaker of mammals, generating a near-24-hour rhythm via an autoregulatory transcriptional-translational feedback loop involving CLOCK, BMAL1, PER, and CRY proteins. Photic input from intrinsically photosensitive retinal ganglion cells (ipRGCs) expressing melanopsin entrains the SCN to the environmental light-dark cycle through the retinohypothalamic tract. The pacemaker role was definitively established by Ralph and colleagues (Science, 1990): grafting SCN tissue from a short-period mutant hamster into SCN-lesioned wild-type hosts restored circadian rhythms with the donor's period. SCN outputs synchronise peripheral clocks in liver, muscle, adipose, and other tissues via neural and humoral signals.

Wake After Sleep Onset (WASO) is the total amount of time spent awake during the night after first sleep onset and before final morning awakening, summing all intra-sleep wake epochs. It is a core polysomnography and actigraphy metric standardised by the American Academy of Sleep Medicine. WASO contributes to the sleep efficiency calculation: SE = total sleep time / time in bed, where total sleep time = time in bed minus sleep-onset latency minus WASO. A WASO above approximately 30 minutes is generally considered clinically significant and is a diagnostic feature of insomnia disorder. WASO rises systematically with age - reflecting more frequent and longer arousals - and is elevated in obstructive sleep apnoea, restless legs syndrome, depression, menopause-related sleep complaints, and shift-work disorder.