# Sleep glossary



- **Total sleep time (TST)** — Minutes actually asleep. The one wearable metric worth trusting.

- **Sleep efficiency** — Time asleep ÷ time in bed. Below ~85% is a common insomnia marker.

- **Sleep latency** — Time to fall asleep after lights-out.

- **WASO** — Wake after sleep onset — total minutes awake during the night.

- **Sleep midpoint** — Halfway between bedtime and wake time; used to measure regularity and social jet lag.

- **Social jet lag** — Difference between weekday and weekend sleep midpoint (Wittmann 2006).

- **Chronotype** — Your biological preference for morning or evening (rMEQ score).

- **Circadian rhythm** — The ~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.

- **Hypopnea** — Partial airway narrowing with reduced airflow and an oxygen dip or arousal.

- **AHI** — Apnea–hypopnea index: events per hour. ≥5 mild, ≥15 moderate, ≥30 severe.

- **Polysomnography (PSG)** — The full sleep-lab study — EEG, breathing, oxygen, muscle activity.

- **SpO₂** — Blood 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.

- **HRV** — Heart-rate variability (SDNN); higher usually means better recovery/parasympathetic tone.

- **CBT-I** — Cognitive behavioural therapy for insomnia — first-line treatment in every guideline.

- **Melatonin** — The 'it's night' hormone. A timing signal, not a sedative; 0.3–0.5 mg is a physiological dose.

- **Sleep inertia** — Grogginess after waking; worse after deep sleep, long naps or overdosed melatonin.

- **Orthosomnia** — Sleep problems driven by preoccupation with tracker data (Baron 2017).

- **First-night effect** — Lighter, more vigilant sleep the first night somewhere new (Tamaki 2016).

- **Adaptive thermogenesis** — The body reducing energy expenditure in response to dieting or high activity.

- **Ghrelin / leptin** — Hunger and satiety hormones; short sleep raises ghrelin and lowers leptin.

- **Cohen's d** — Effect size: 0.2 small, 0.5 medium, 0.8 large. Used in Lab experiment results.

- **Pearson r** — Correlation 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.

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