Sleep Science Glossary

A complete reference covering sleep architecture, circadian biology, neuroscience, natural supplements, sleep disorders, and clinical assessment metrics. Each definition is written for clarity without sacrificing scientific precision.

Circadian Biology

Circadian Rhythm

An internally generated biological cycle of approximately 24 hours that governs timing of sleep, hormone release, metabolism, body temperature, and cell division. Present in virtually every cell in the human body, not just the brain.

Suprachiasmatic Nucleus (SCN)

A paired structure in the hypothalamus containing approximately 20,000 neurons that serves as the master circadian pacemaker. The SCN receives direct light input via the retinohypothalamic tract and coordinates all peripheral circadian clocks throughout the body.

Dim Light Melatonin Onset (DLMO)

The time point at which melatonin levels rise above a threshold under standardised dim light conditions. DLMO is considered the most reliable marker of the circadian phase and occurs approximately 2 hours before habitual sleep time.

Chronotype

An individual's intrinsic preference for sleep and wake timing, reflecting their underlying circadian period length. Chronotype has a strong genetic basis and shifts across the lifespan - teenagers become more evening-typed; older adults shift morning.

Zeitgeber

German for 'time giver' - any external cue that entrains (synchronises) the circadian clock to the 24-hour day. The strongest zeitgeber is light; others include meal timing, exercise, social interaction, and temperature.

Entrainment

The process by which an external zeitgeber synchronises the internal circadian clock to the 24-hour environmental cycle. Light entrainment occurs via melanopsin-containing intrinsically photosensitive retinal ganglion cells that project directly to the SCN.

Circadian Misalignment

A state in which the internal circadian phase is desynchronised from the external environment or from social/work schedules. Causes include shift work, jet lag, and social jetlag. Associated with metabolic risk, mood disorders, and impaired immune function.

Social Jetlag

The discrepancy between an individual's biological sleep timing (determined by chronotype) and the sleep timing imposed by social and occupational schedules. Evening chronotypes forced onto early schedules experience chronic social jetlag.

Sleep Architecture

Sleep Architecture

The structural pattern of sleep stages across a night, typically represented as a hypnogram. Normal adult sleep architecture consists of 4-6 sleep cycles of approximately 90 minutes each, with earlier cycles weighted toward slow-wave sleep.

Sleep Cycle

A single progression through NREM stages (N1-N2-N3) followed by REM sleep, averaging approximately 90 minutes. Adults experience 4-6 cycles per night. The proportion of deep sleep (N3) is highest in the first two cycles.

NREM Sleep

Non-rapid eye movement sleep - the phases of sleep that do not involve rapid eye movements. Comprises three stages (N1, N2, N3) progressing from light to deep sleep. NREM sleep is associated with physical restoration and memory consolidation of declarative memories.

REM Sleep

Rapid eye movement sleep - characterised by rapid conjugate eye movements, near-complete skeletal muscle atonia, and brain activity resembling wakefulness. REM sleep is associated with emotional memory processing, creativity, and procedural memory consolidation.

N3 Sleep / Slow-Wave Sleep

The deepest NREM sleep stage, characterised by high-amplitude delta waves comprising at least 20% of the epoch. Associated with physical restoration, growth hormone secretion, immune activation, and metabolic regulation.

Sleep Spindles

Bursts of oscillatory brain activity (12-15 Hz) lasting 0.5-3 seconds that characterise N2 sleep. Sleep spindles are associated with memory consolidation - spindle density correlates with overnight learning improvement.

Sleep Efficiency

The percentage of time in bed actually spent asleep: (Total Sleep Time / Time in Bed) x 100. Normal sleep efficiency in healthy adults is above 85%. Below 85% is clinically considered poor sleep efficiency.

Glymphatic System

A brain-specific waste clearance system using cerebrospinal fluid to flush metabolic waste from brain tissue. Most active during deep NREM sleep. Clears amyloid-beta and tau - proteins implicated in Alzheimer's disease. Chronic poor sleep impairs glymphatic clearance.

Sleep Pressure and Regulation

Two-Process Model

The Borbely model proposing that sleep timing and intensity are controlled by two processes: Process S (homeostatic sleep pressure, building during wakefulness) and Process C (circadian alerting signal). Their interaction determines when we feel sleepy and when alert.

Adenosine

A purine nucleoside that accumulates in the brain during wakefulness, serving as the primary molecular mediator of homeostatic sleep pressure. Caffeine blocks adenosine receptors, temporarily masking sleep pressure without eliminating it.

Sleep Debt

The cumulative deficit of sleep relative to an individual's biological sleep need. Partial sleep debt impairs cognition at rates comparable to total sleep deprivation but with limited subjective awareness.

Sleep Inertia

The transitional state of grogginess and impaired performance occurring immediately after waking, particularly when woken from deep (N3) sleep. Adenosine clearance and reestablishment of alerting signal takes 15-60 minutes.

Sleep Homeostasis

The regulatory mechanism that maintains the balance of sleep need over time. After sleep deprivation, sleep homeostasis drives compensatory increases in sleep duration and depth. This self-correcting system prevents chronic sleep extremes.

Insomnia and Sleep Onset

Insomnia Disorder

A clinical condition defined by difficulty initiating or maintaining sleep, early morning awakening, or non-restorative sleep, occurring at least 3 nights per week for at least 3 months, causing significant daytime impairment.

Sleep Onset Insomnia

Difficulty falling asleep at the beginning of the night, characterised by prolonged sleep onset latency (>30 minutes). Causes include physiological arousal (elevated cortisol), anxiety, delayed circadian phase, and inadequate sleep pressure.

Sleep Maintenance Insomnia

Difficulty staying asleep through the night, characterised by frequent or prolonged awakenings after initially falling asleep. More common in older adults as sleep architecture becomes lighter.

Hyperarousal

An elevated baseline level of physiological and cognitive activation that predisposes to and perpetuates insomnia. Manifestations include elevated 24-hour cortisol, increased metabolic rate during sleep, and ruminative thought patterns.

Conditioned Arousal

A learned association between the bedroom environment and wakefulness rather than sleep, which develops when prolonged wakefulness in bed creates classical conditioning. Central mechanism in chronic insomnia.

Sleep Neuroscience

Orexin (Hypocretin)

A neuropeptide produced by approximately 80,000 neurons in the lateral hypothalamus that strongly promotes wakefulness. Loss of orexin neurons causes narcolepsy. Orexin receptor antagonists (suvorexant, lemborexant) block orexin signalling to promote sleep.

VLPO (Ventrolateral Preoptic Area)

The primary sleep-promoting nucleus in the anterior hypothalamus, containing GABAergic neurons that inhibit arousal centres to initiate and maintain sleep. VLPO neurons are maximally active during sleep.

Ascending Arousal System

The network of brainstem and hypothalamic nuclei that promotes and maintains wakefulness. Components include the locus coeruleus (norepinephrine), raphe nuclei (serotonin), and lateral hypothalamus (orexin). All are inhibited by VLPO neurons during sleep.

Flip-Flop Switch Model

A model of sleep-wake control comparing the switch to an electronic flip-flop circuit. VLPO and arousal neurons mutually inhibit each other, creating a bistable system that switches rapidly between stable wake and stable sleep states.

Sleep Hormones and Neurotransmitters

Melatonin

A hormone produced by the pineal gland in response to darkness, acting as the primary endogenous signal of nocturnal timing. Melatonin does not induce sleep directly but signals circadian phase. Serum levels rise approximately 2 hours before habitual sleep onset.

Cortisol

The primary glucocorticoid stress hormone with a strong circadian rhythm: lowest in the middle of the night, rising sharply in the hour before waking. Chronic psychological stress elevates evening cortisol, directly interfering with sleep onset.

GABA

The primary inhibitory neurotransmitter of the central nervous system. GABA reduces neural excitability by hyperpolarising neurons. The VLPO's sleep-promoting neurons are GABAergic. Most pharmacological sleep aids (benzodiazepines, Z-drugs) potentiate GABA-A receptor activity.

Glutamate

The primary excitatory neurotransmitter. Glutamate activity must be reduced for sleep initiation. NMDA-type glutamate receptors are voltage-dependently blocked by magnesium - a key mechanism by which magnesium supplementation reduces neural excitability.

Growth Hormone (GH)

A peptide hormone with its largest secretory pulse tightly linked to slow-wave sleep onset. GH promotes tissue repair, protein synthesis, and fat metabolism. Sleep deprivation substantially reduces GH secretion, impairing recovery.

Natural Sleep Supplements

Magnesium Glycinate

A chelated compound of magnesium and glycine with high bioavailability (40-60%). Promotes sleep via dual mechanisms: magnesium activates GABA-A receptors and blocks NMDA glutamate receptors; glycine lowers core body temperature by dilating peripheral blood vessels - a reliable physiological sleep onset trigger.

Magnesium L-Threonate (MgT)

A magnesium form chelated to L-threonic acid that crosses the blood-brain barrier more efficiently than other forms, increasing cerebrospinal fluid magnesium concentrations. Developed for neurological applications including cognitive support and sleep architecture enhancement.

L-Theanine

A non-protein amino acid found almost exclusively in tea leaves. Crosses the blood-brain barrier and increases alpha brain wave activity (8-13 Hz) - the neural signature of relaxed alertness - within 40 minutes of ingestion. Does not cause sedation; creates 'alert calm'.

Ashwagandha (Withania somnifera)

An adaptogenic herb used for stress, anxiety, and sleep. KSM-66 and Sensoril are standardised root extracts with the most clinical trial evidence. Reduces cortisol in chronic stress contexts and shortens sleep onset latency. Not a sedative - reduces HPA axis hyperactivity that impairs sleep.

KSM-66

A patented full-spectrum ashwagandha root extract standardised to minimum 5% withanolides. The most studied ashwagandha extract with RCT evidence for cortisol reduction, sleep quality improvement, and anxiety reduction.

Glycine

A simple amino acid that acts as an inhibitory neurotransmitter in the brainstem and reduces core body temperature by promoting peripheral vasodilation - a reliable trigger for sleep onset. 3g glycine before bed improves subjective sleep quality in RCTs.

5-HTP

The direct precursor to serotonin in the brain, produced from tryptophan. Crosses the blood-brain barrier efficiently. At 50-100mg before bed, increases brain serotonin, supporting NREM deep sleep architecture and downstream melatonin production.

Valerian Root

A perennial herb whose root contains valerenic acid, which modulates GABA-A receptors. The clinical evidence is inconsistent across trials, reflecting variable extract quality. Standardised extracts (0.8% valerenic acid minimum) show more consistent sleep latency improvements.

Sleep Disorders

Sleep Apnoea

A disorder characterised by repeated episodes of partial or complete upper airway obstruction during sleep, causing intermittent hypoxia and sleep fragmentation. Affects approximately 4% of adults. Treatment is CPAP for moderate-severe OSA.

Restless Legs Syndrome (RLS)

A neurological sensorimotor disorder characterised by an irresistible urge to move the legs, worsened at rest and in the evening, and relieved by movement. Affects 5-10% of adults; strongly associated with iron deficiency and dopamine dysfunction.

Delayed Sleep Phase Syndrome (DSPS)

A circadian rhythm disorder in which the intrinsic circadian phase is chronically delayed, resulting in inability to sleep before 2-6am and difficulty waking before midday. Treatment involves strategically timed light therapy and low-dose melatonin.

Narcolepsy

A chronic neurological disorder caused by destruction of orexin-producing neurons in the lateral hypothalamus, resulting in excessive daytime sleepiness, cataplexy, sleep paralysis, and hypnagogic hallucinations.

Sleep Assessment and Metrics

Polysomnography (PSG)

The gold standard laboratory sleep study recording EEG, EOG, EMG, ECG, respiratory effort, oxygen saturation, and leg movements. Used to diagnose sleep apnoea, narcolepsy, REM behaviour disorder, and other sleep disorders.

Sleep Onset Latency (SOL)

The time from lights out to the first epoch of sleep. Normal SOL in adults is 10-20 minutes. Consistent SOL above 30 minutes is clinically significant. SOL is reduced by adequate homeostatic sleep pressure, appropriate circadian timing, and certain supplements.

WASO

Wakefulness After Sleep Onset - total time spent awake after initially falling asleep. Normal WASO is under 30 minutes. Elevated WASO is the defining metric of sleep maintenance insomnia.

Pittsburgh Sleep Quality Index (PSQI)

A validated 19-item self-report questionnaire assessing sleep quality over the previous month, yielding a global score from seven components. A global score above 5 indicates poor sleep quality. Widely used in clinical trials.

Actigraphy

A wrist-worn device measuring movement to estimate sleep-wake cycles over days to weeks. Less accurate than PSG for sleep staging but provides valuable longitudinal data on sleep patterns and circadian timing.