Sleep glossary

Total sleep time (TST)Minutes actually asleep. The one wearable metric worth trusting.Sleep efficiencyTime asleep ÷ time in bed. Below ~85% is a common insomnia marker.Sleep latencyTime to fall asleep after lights-out.WASOWake after sleep onset — total minutes awake during the night.Sleep midpointHalfway between bedtime and wake time; used to measure regularity and social jet lag.Social jet lagDifference between weekday and weekend sleep midpoint (Wittmann 2006).ChronotypeYour biological preference for morning or evening (rMEQ score).Circadian rhythmThe ~24 h internal clock, set mainly by light.Sleep pressure (adenosine)The drive to sleep that builds while awake; caffeine blocks adenosine receptors.Obstructive sleep apnea (OSA)Repeated airway collapse during sleep → pauses in breathing, oxygen dips, arousals.HypopneaPartial airway narrowing with reduced airflow and an oxygen dip or arousal.AHIApnea–hypopnea index: events per hour. ≥5 mild, ≥15 moderate, ≥30 severe.Polysomnography (PSG)The full sleep-lab study — EEG, breathing, oxygen, muscle activity.SpO₂Blood oxygen saturation. Dips below ~90% at night can indicate breathing events.Resting heart rate (RHR)Heart rate at rest; lower with fitness and calm — Galpin's canary.HRVHeart-rate variability (SDNN); higher usually means better recovery/parasympathetic tone.CBT-ICognitive behavioural therapy for insomnia — first-line treatment in every guideline.MelatoninThe 'it's night' hormone. A timing signal, not a sedative; 0.3–0.5 mg is a physiological dose.Sleep inertiaGrogginess after waking; worse after deep sleep, long naps or overdosed melatonin.OrthosomniaSleep problems driven by preoccupation with tracker data (Baron 2017).First-night effectLighter, more vigilant sleep the first night somewhere new (Tamaki 2016).Adaptive thermogenesisThe body reducing energy expenditure in response to dieting or high activity.Ghrelin / leptinHunger and satiety hormones; short sleep raises ghrelin and lowers leptin.Cohen's dEffect size: 0.2 small, 0.5 medium, 0.8 large. Used in Lab experiment results.Pearson rCorrelation strength from −1 to 1. Correlation ≠ causation — hence the Lab.

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Educational summary of published research, not medical advice. Screening scores are not diagnoses. If a score is elevated or you feel unwell, talk to a clinician.