😴 Sleep Science Facts

Adenosine accumulates in basal forebrain during wakefulness, inhibiting arousal neurons; extracellular adenosine doubles after 6h awake; caffeine blocks A1/A2A receptors without reducing adenosine levels.

80% of REM awakenings report vivid dreaming; NREM awakenings also report dreamlike mentation 50% of the time; the amygdala is hyperactive during REM, explaining the emotional intensity of most dreams.

Glymphatic CSF flow increases ~60% during NREM sleep via aquaporin-4 channels on astrocytic endfeet; this clears amyloid-beta and tau; chronic sleep deprivation elevates brain amyloid-beta measurably within 24h.

70–80% of daily growth hormone is secreted during the first slow-wave sleep episode; GH pulse amplitude is 5–10× higher than daytime pulses; total sleep deprivation abolishes the nocturnal GH surge entirely.

Hypnagogic hallucinations affect 25–37% of the general population; they occur during N1 sleep onset as dream-like imagery intrudes into fading wakefulness; visual hallucinations are most common (86% of reports).

Lucid dreaming was scientifically verified by Hearne (1975) and LaBerge (1980) using pre-agreed eye movement signals during REM; 55% of people experience it at least once; ~23% report monthly frequency.

Hippocampal sharp-wave ripples during NREM replay learned sequences to the neocortex for long-term storage; REM sleep consolidates procedural and emotional memories; deprivation impairs retention 20–40%.

Slow-wave sleep declines from ~20% in young adults to <5% by age 60; WASO increases ~28 minutes per decade; circadian phase advances ~1h per decade; sleep efficiency declines from ~95% to ~80%.

Women average 30% more slow-wave sleep than men throughout adulthood; women have 1.4× higher insomnia prevalence than men; menopause reduces sleep efficiency by 10–15%; testosterone suppresses SWS in men.

Newborns sleep 14–17h/day with 50% in active REM sleep; sleep cycles are 50–60 min (vs 90 min adults); circadian rhythm develops by 3–6 months when the SCN becomes functionally responsive to light.

Alcohol (0.5–1g/kg) reduces REM sleep in the first half of the night by 20–40%; rebound REM fragmentation occurs in the second half; slow-wave sleep quality is also impaired despite feeling like a sedative.

480nm blue-green light maximally activates melanopsin in ipRGC retinal cells; 2h of LED screen exposure before bed delays melatonin onset by 90 minutes and reduces REM sleep the following night.

200mg caffeine taken 6h before bed reduces total sleep time by ~1 hour; caffeine half-life is 5–6h via CYP1A2; it blocks A1/A2A adenosine receptors, masking sleep pressure that rebounds upon its clearance.

Core body temperature must fall 1–2°C for sleep onset; optimal bedroom temperature is 18–20°C; a warm bath 1–2h before sleep speeds this cooling via skin vasodilation, reducing sleep onset latency.

Altitude above 2,500 m disrupts sleep architecture via hypoxia-driven periodic breathing (Cheyne-Stokes respiration); SWS decreases up to 40% and arousals double within the first 2 nights at 3,500 m.

WHO Night Noise Guidelines (2009) recommend <40 dB(A) Lnight to prevent sleep disruption; EEG arousals occur reliably at peaks above 35 dB(A) during NREM sleep and at lower thresholds in REM; chronic exposure >55 dB(A) increases cardiovascular disease risk significantly.

CBT-I is the first-line recommended treatment for chronic insomnia; meta-analyses of 80+ RCTs show 50–70% remission rates, superior to hypnotic medications at 6- and 12-month follow-up.

Chronotypes reflect genetically determined circadian phase; evening types show peak cognitive performance 2–4h later than morning types; PER3 length polymorphism affects chronotype and sleep architecture.

The circadian clock runs at ~24.2 hours endogenously; the suprachiasmatic nucleus resets it daily via retinal light input; disruption of this rhythm increases metabolic disease risk by 20–40%.

Cortisol rises 50–160% within 30 minutes of waking — the cortisol awakening response (CAR); it is HPA-axis driven, distinct from the circadian cortisol rhythm, and requires intact sleep quality to be robust.

Jet lag resynchronizes at ~1 day per time zone eastward and 1.5 days westward; eastward travel is harder because the clock must advance against its natural ~24.2-hour free-running period.

Melatonin secretion begins ~2 hours before sleep, peaks at 2–4am with a 10-fold rise from daytime baseline; 2 hours of 480nm blue light exposure delays this onset by 90 minutes or more.

The suprachiasmatic nucleus contains ~20,000 neurons in bilateral hypothalamic clusters; SCN lesions abolish circadian rhythms in rats; it receives direct photic input via the retinohypothalamic tract.

Insomnia involves chronic hyperarousal: elevated 24-hour cortisol, higher metabolic rate, and increased high-frequency EEG activity at night; CBT-I achieves remission in 70–80% of patients and outperforms sleep medication.

10–20 minute naps improve alertness up to 100% without sleep inertia; NASA studied pilots given 40-minute naps and found 34% improved performance and 100% more alertness; 90-min naps include full cycle with REM.

Sleepwalking and sleep terrors arise from incomplete arousal during N3 slow-wave sleep; prevalence 3–4% in adults; 60% monozygotic twin concordance indicates strong genetic basis; typically resolve in adulthood.

Biphasic sleep (single nighttime period + afternoon nap) has pre-industrial historical evidence; extreme polyphasic schedules lack sufficient SWS and REM; core sleep need is 6–8 hours regardless of distribution.

Obstructive sleep apnea affects 26% of adults aged 30–70; AHI >30 events/hour is severe; each apnea event causes 10–90s hypoxia, arousal, and sympathetic surge; CPAP reduces CVD risk and blood pressure.

Sleep paralysis affects 7.6% of the general population; it represents persistence of REM muscle atonia into wakefulness; hallucinations occur in ~75% of episodes; linked to sleep deprivation and disrupted schedules.

K-complexes are high-amplitude biphasic waveforms during N2 sleep, exceeding 75μV over 0.5 seconds; they represent the largest single electrical event in the healthy human EEG and protect sleep from arousal.

REM sleep comprises 20–25% of total sleep; the amygdala is highly active during REM, supporting emotional memory processing; REM atonia prevents acting out dreams via brainstem inhibition.

Each sleep cycle lasts ~90 minutes; 4–6 cycles per night; early cycles are slow-wave sleep dominant and late cycles are REM dominant, serving distinct cognitive and physical functions.

Sleep spindles are 12–15Hz thalamic oscillations lasting 0.5–2 seconds during N2; individuals with more spindles perform better on declarative memory tests; ~1,000–2,000 spindles occur per night.

Human sleep has four stages: N1 (5%), N2 (50%), N3 slow-wave (20–25%), and REM (20–25%); a complete cycle is ~90 minutes, with 4–6 cycles per night.

Slow-wave sleep (N3) produces 70–80% of nightly growth hormone secretion during the first episode; delta waves <2Hz mark this deepest stage comprising 20–25% of total sleep.

Microsleep episodes last 0.5–15 seconds without awareness; at 100km/h, a 10-second microsleep covers 280m unguided; risk is highest in monotonous tasks and equivalent to 0.08% BAC driving impairment after 18h awake.

Sleeping <6h is associated with 20% higher CVD risk; sleep deprivation elevates blood pressure acutely by 5–10 mmHg; non-dipping nocturnal blood pressure pattern signals elevated cardiovascular risk.

One night of sleep deprivation reduces NK cell activity by 28%; sleeping <6h reduces hepatitis B vaccine antibody response 11.5× compared to 7+ hour sleepers; IL-6 and TNF-α peak during NREM sleep.

U-shaped mortality curve: 7–8h sleep is optimal; <6h sleep is associated with 12% higher all-cause mortality and >9h with 30% higher mortality in meta-analyses of 1.3 million participants across 16 studies.

One week of 5h/night sleep reduces leptin 18% and increases ghrelin 28%, creating a 24% increase in hunger; subjects preferentially craved high-carbohydrate, calorie-dense foods.

2 weeks of 6h/night produces psychomotor deficits equivalent to 48h total sleep deprivation; subjective sleepiness stabilizes after 7 days while objective impairment continues worsening; full recovery requires 1+ week.

17–19 hours awake impairs psychomotor performance equivalent to 0.05% blood alcohol; after 24h without sleep, reaction time doubles; 2 weeks of 6h/night equals 48h total sleep deprivation.

NBA players with >8h sleep per night scored 9% more points and had faster reaction times; 5–7 weeks of sleep extension to 10h improved sprint speed 4% and shooting accuracy 9% in NCAA basketball players.

Narcolepsy type 1 is caused by autoimmune destruction of 70,000–90,000 orexin-producing hypothalamic neurons; CSF hypocretin-1 falls below 110 pg/mL (normal >200 pg/mL) and MSLT shows mean sleep latency <8 min with ≥2 SOREMPs.

RLS is characterized by urge to move legs at rest, with 80% of patients showing periodic limb movements during sleep (PLMS) every 20–40 seconds; ferritin below 50 ng/mL is linked to symptom severity and IV iron supplementation reduces PLMS by 70%.

Newborns spend 50% of sleep time in active (REM) sleep, declining to 30% by age 2 and 20–25% by adolescence; slow-wave sleep proportion peaks in childhood (35–40%) and halves by age 60, reflecting the synaptic pruning and plasticity demands of developing brains.

Night shift work reduces sleep duration 1–4 h/day versus day workers; cardiovascular disease risk increases 40%, type 2 diabetes risk doubles, and chronic circadian disruption is classified as a probable carcinogen (IARC Group 2A) based on epidemiologic and mechanistic evidence.

Benzodiazepines reduce sleep latency by ~10 min and increase TST by 30–40 min but suppress slow-wave sleep 25–30% and REM 15–20%; orexin receptor antagonists (suvorexant, lemborexant) preserve natural sleep architecture and avoid hangover effects.

Lateral sleeping position enhances glymphatic waste clearance ~25% over supine in rodent models; supine position increases snoring episodes 3–4× and doubles apnea-hypopnea index versus lateral in OSA patients; fetal/lateral positions are most common in adults (~54%).

Polysomnography (PSG) remains the gold standard for sleep staging with trained technologist scoring; consumer wearables show 85–95% sleep/wake accuracy but overestimate total sleep time by 30–60 min and struggle with N1/N2 discrimination.