Sleep

Actigraphy uses a wrist-worn motion sensor (accelerometer) to estimate your sleep and wake. It reads your movement patterns over days to weeks. It is a low-burden, take-home…

Adenosine is a small molecule (a purine nucleoside) that builds up in your brain while you are awake, as a byproduct of neurons using energy. It acts on receptors (A1 and A2A) to…

Chronotype is your individual leaning toward earlier or later sleep-wake timing. It is commonly described as a morning type, an intermediate type, or an evening type. It is…

Your circadian rhythm is the body's roughly 24-hour internal clock. It coordinates when you sleep and wake, when hormones release, and your body temperature and metabolism. It is…

The cortisol awakening response (CAR) is a sharp rise in your salivary cortisol right after you wake up. On average it climbs about 50% (commonly reported between roughly 38 and…

Deep sleep, also called slow-wave sleep (N3), is marked by high-amplitude 'delta' waves on the EEG, and the highest arousal threshold (it is the hardest stage to wake you from).…

Dim Light Melatonin Onset (DLMO) is a specific time in the evening. It is when your body's own melatonin, in saliva or plasma, rises above a set threshold. The measurement is…

The glymphatic system, described by Iliff, Nedergaard, and colleagues in 2012, is your brain's waste-clearance pathway. Cerebrospinal fluid flows along the spaces around blood…

Insomnia is a sleep disorder: persistent trouble falling asleep, staying asleep, or getting restful sleep, despite having enough time to sleep, and it causes real daytime…

Melatonin is a hormone secreted by your pineal gland in response to darkness. It signals biological night and helps align your circadian system. It helps you fall asleep,…

Orexin-A and orexin-B are two excitatory brain signals (neuropeptides). They are also called hypocretin-1 and hypocretin-2. A small cluster of neurons in your hypothalamus makes…

Polysomnography (PSG) is the gold-standard sleep study, done overnight in a lab. It records many channels at once: brain waves (EEG), eye movements (EOG), muscle activity (EMG),…

REM sleep is named for its rapid eye movements. It is a distinct sleep stage with four signatures. Your eyes dart quickly. You dream vividly. Your brain activity is near-waking.…

Sleep apnea is a disorder of repeated breathing pauses or shallow breaths (apneas and hypopneas) during sleep. The most common form is obstructive sleep apnea, from your upper…

Sleep architecture is how your sleep stages are organized across the night. A typical night runs through four to six 90-minute cycles. Each cycle moves through N1, N2, N3 (deep,…

Sleep debt is the running shortfall between the sleep your body needs and the sleep you actually get, night after night. It builds up bit by bit. Restricting sleep to 6 hours a…

Sleep efficiency is the percentage of your time in bed that you actually spend asleep. You calculate it as total sleep time divided by time in bed. In adults, a value of 85% or…

Sleep latency is how long it takes you to fall asleep, from lights-out to the first moment of sleep. It is usually measured in minutes during a sleep study (polysomnography). A…

The two-process model, proposed by Alexander Borbély in 1982, explains sleep and wake as two forces working together. Process S is homeostatic sleep pressure. It builds up while…

Sleep regularity is how consistent your sleep-wake timing is from day to day. It is scored by the Sleep Regularity Index (SRI), which runs from −100 to 100. A 0 means totally…

Sleep spindles are short bursts of rhythmic brain activity, in the 11-16 Hz range. On an EEG, they look like waxing-and-waning 'sigma-band' waves, and they define stage N2 of…

Social jetlag is the chronic mismatch between your internal body clock and the sleep-wake schedule your obligations (work, school) impose. It is measured as the difference in the…

The suprachiasmatic nucleus (SCN) is a paired structure in your hypothalamus, sitting just above the optic chiasm, with about 20,000 neurons per side. It is the master circadian…

Wake After Sleep Onset (WASO) is the total time you spend awake during the night. It counts the minutes after you first fall asleep, but before your final wake-up. It sums up all…