Glossary A-Z
Sleep Terminology, Defined
50 sleep medicine terms in plain language — from AHI to Vagal Tone. Use this as a reference whenever you encounter sleep jargon you don't recognize.
A
- Adenosine
- A neurochemical that builds up in the brain during wakefulness, producing the sensation of sleep pressure. Caffeine works by blocking adenosine receptors.
- AHI (Apnea-Hypopnea Index)
- The number of apnea and hypopnea breathing events per hour of sleep. 5-15 is mild sleep apnea, 15-30 is moderate, 30+ is severe. Read more →
- Apnea
- A pause in breathing during sleep lasting 10+ seconds. Caused by partial or complete collapse of the upper airway.
B
- Bruxism
- Teeth grinding or jaw clenching during sleep. Often associated with mild airway obstruction rather than stress alone. Read more →
- Box Breathing
- 4-4-4-4 breathing pattern (inhale, hold, exhale, hold) used to activate the parasympathetic nervous system. Popularized by Navy SEALs. Read more →
C
- CBT-I
- Cognitive Behavioral Therapy for Insomnia. The first-line evidence-based treatment for chronic insomnia, more effective long-term than sleeping pills.
- Chronotype
- Your genetic predisposition toward morning or evening alertness. Roughly 60% heritable. Larks, owls, and intermediates. Read more →
- Circadian Rhythm
- The internal 24-hour body clock that regulates sleep, hormone release, body temperature, and other functions. Synchronized primarily by light.
- Cortisol
- A stress hormone that follows a daily curve — high in morning, low overnight. Elevated overnight cortisol fragments sleep.
- CPAP
- Continuous Positive Airway Pressure. The gold-standard treatment for moderate-to-severe obstructive sleep apnea.
D
- Deep Sleep (N3)
- The slow-wave sleep stage where physical recovery happens — growth hormone pulses, immune consolidation, glymphatic clearance. Read more →
- Delayed Sleep Phase
- A circadian rhythm pattern where the natural sleep window is shifted later than socially desired. Common in strong evening chronotypes.
- DLMO (Dim Light Melatonin Onset)
- The time of evening when melatonin secretion begins, typically 2-3 hours before natural sleep onset. The most reliable marker of circadian phase.
G
- GABA
- The brain's primary inhibitory neurotransmitter. Sleep medications like benzodiazepines act on GABA receptors. Magnesium supports GABA signaling.
- Glycine
- An inhibitory amino acid that promotes slow-wave sleep at 3g taken 30 min before bed. Read more →
- Glymphatic System
- The brain's waste-clearance system that flushes metabolic byproducts during deep sleep, including amyloid beta.
H
- HRV (Heart Rate Variability)
- The variation between heartbeats. Higher HRV = better parasympathetic recovery. The single most useful daily wearable metric. Read more →
- Hypnic Jerk
- Involuntary muscle twitches at sleep onset. Common, benign, often triggered by caffeine or stress.
- Hypopnea
- A partial reduction in breathing during sleep — airflow drops at least 30% but doesn't fully stop.
I
- Insomnia
- Difficulty falling asleep, staying asleep, or waking too early, persistent for 3+ nights/week for 3+ months. Treated first-line with CBT-I.
J
- Jet Lag
- The temporary mismatch between internal circadian rhythm and external time zone after travel. Recovery roughly 1 day per time zone crossed.
L
- L-Theanine
- An amino acid in tea that promotes alpha-wave activity and parasympathetic relaxation. 100-200mg before bed. Read more →
- Lark (Morning Type)
- A chronotype with natural early bedtime and early wake. Roughly 15-20% of adults.
M
- Magnesium Glycinate
- A well-absorbed chelated form of magnesium. Supports GABA and parasympathetic activation. The right form for sleep support. Read more →
- Melatonin
- A hormone released by the pineal gland in the evening that signals nighttime to the body. Suppressed by light exposure. Read more →
- Micro-Arousal
- A brief sympathetic spike during sleep — typically not consciously noticed — that fragments architecture. Often driven by airway events.
- Mouth Breathing
- Breathing through the mouth during sleep, which fragments sleep architecture, dries airways, and is associated with snoring and bruxism. Read more →
- Mouth Taping
- A mechanical intervention — a strip of skin-safe tape across the lips at night — that prevents mouth breathing during sleep. Read more →
N
- N1 / N2 / N3
- The three stages of non-REM sleep. N1 is light transition, N2 is the bulk of sleep, N3 is deep slow-wave sleep.
- Nasal Breathing
- Breathing exclusively through the nose, which produces nitric oxide, humidifies the air, and supports parasympathetic activation. Read more →
- Nasal Strips
- Adhesive strips applied to the bridge of the nose that mechanically dilate the nasal passage, easing nasal breathing. Read more →
- Nitric Oxide
- A molecule produced in the nasal sinuses during nasal breathing that improves oxygen uptake and supports vascular function.
- Nocturia
- Frequent nighttime urination that disrupts sleep. Often caused by late-evening hydration; worth medical workup if chronic.
O
- OSA (Obstructive Sleep Apnea)
- Repeated airway collapse during sleep producing breathing stoppages. Treated primarily with CPAP. Associated with cardiovascular disease.
- Owl (Evening Type)
- A chronotype with natural late bedtime and late wake. Roughly 10-15% of adults are strong owls.
P
- Parasympathetic Nervous System
- The "rest and digest" branch of the autonomic nervous system. Activated by slow breathing, side-lying, and nasal breathing.
- Phase Advance / Phase Delay
- Shifting your circadian rhythm earlier (advance) or later (delay). Morning light advances; evening light delays.
R
- REM (Rapid Eye Movement) Sleep
- The dream-heavy sleep stage where emotional memory consolidates. Suppressed by alcohol and many medications.
- Resting Heart Rate (RHR)
- Heart rate at full rest. Overnight RHR is the most informative measurement, captured by wearables. Read more →
S
- Sleep Architecture
- The pattern of sleep stages across the night. Healthy architecture has multiple complete cycles with appropriate proportions of N3 and REM.
- Sleep Cycle
- A ~90-minute progression through light sleep → deep sleep → REM. Adults complete 4-6 cycles per night. Read more →
- Sleep Debt
- Cumulative shortfall from adequate sleep. Cannot be fully recovered with a single long sleep; requires multiple nights of adequate rest.
- Sleep Inertia
- The groggy, foggy state immediately after waking, especially from deep sleep. Worse when alarm wakes you mid-cycle.
- Sleep Latency
- The time from getting into bed to falling asleep. Normal: 10-20 minutes. Read more →
- Sleepmaxxing
- The protocol-based optimization of sleep using a stack of evidence-based interventions. Read more →
- Slow-Wave Sleep
- Same as deep sleep (N3). Named for the slow, synchronized brain waves observed on EEG.
- STOP-BANG
- An 8-question screening tool for obstructive sleep apnea risk. Score 3+ warrants a sleep study.
- Sympathetic Nervous System
- The "fight or flight" branch of the autonomic nervous system. Elevated overnight sympathetic tone fragments sleep.
T
- Tryptophan
- An amino acid precursor to serotonin and melatonin. Found in turkey, dairy, pumpkin seeds. Modest direct sleep effect.
V
- Vagal Tone / Vagus Nerve
- The vagus nerve is the primary conduit of parasympathetic signaling. Slow exhales and cold exposure increase vagal tone.
W
- Wake After Sleep Onset (WASO)
- The time spent awake after initially falling asleep. Healthy WASO is under 20 minutes per night for adults.