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…

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…

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,…

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…

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…

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…

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…

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…

Insomnia is a sleep disorder defined by persistent difficulty initiating or maintaining sleep — or non-restorative sleep — despite adequate sleep opportunity, causing clinically…

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,…

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…

Polysomnography (PSG) is the gold-standard multi-channel sleep study conducted in a laboratory setting, simultaneously recording electroencephalography (EEG), electrooculography…

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…

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…

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,…

Sleep debt is the cumulative shortfall between an individual's biologically required sleep duration and the sleep actually obtained over consecutive nights. It builds…

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…

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…

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…

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…

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…

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,…

The suprachiasmatic nucleus (SCN) is a paired hypothalamic structure situated immediately above the optic chiasm, containing approximately 20,000 neurons per side. It functions…

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…