Sleep Science Glossary | Sleep University by WantMattress

Adenosine

A chemical that accumulates in the brain during waking hours and creates sleep pressure — the growing urge to sleep. The longer you stay awake, the more adenosine builds up. Sleep clears adenosine, which is why you feel refreshed after a good night's rest. Caffeine works by blocking adenosine receptors, temporarily masking sleepiness without actually reducing sleep debt.

Chronotype

Your natural preference for when you feel most alert and when you prefer to sleep. Often described as "morning lark" or "night owl." Chronotype is largely genetic and changes across the lifespan — teenagers naturally shift towards later sleep times, while older adults tend to prefer earlier schedules. Understanding your chronotype helps you align your sleep schedule with your biology.

Circadian Rhythm

The body's internal 24-hour clock that regulates the sleep-wake cycle, hormone release, body temperature, and other biological functions. Your circadian rhythm is primarily controlled by the suprachiasmatic nucleus (SCN) in the hypothalamus and is synchronised to the external light-dark cycle through a process called entrainment. Disruption to circadian rhythm — from shift work, jet lag, or irregular schedules — is associated with poor sleep quality and increased health risks.

Melatonin

A hormone produced by the pineal gland that signals to the body that it is time to sleep. Melatonin production increases in the evening as light levels drop and decreases in the morning with exposure to bright light. Blue light from screens can suppress melatonin production, which is one reason experts recommend avoiding screens in the hour before bed. Melatonin helps regulate sleep timing but does not generate sleep itself.

NREM Sleep (Non-Rapid Eye Movement)

The three stages of sleep that are not REM sleep. Stage N1 is the lightest transition from wakefulness. Stage N2 makes up about 50% of total sleep time and involves sleep spindles that help consolidate memory. Stage N3 is deep sleep (also called slow-wave sleep), which is critical for physical restoration, immune function, and growth hormone release. Most deep sleep occurs in the first half of the night.

REM Sleep (Rapid Eye Movement)

A stage of sleep characterised by rapid eye movements, vivid dreaming, and temporary muscle paralysis (atonia). REM sleep is essential for emotional processing, memory consolidation, and cognitive function. REM periods get longer as the night progresses, with the longest occurring in the final hours of sleep. This is why cutting sleep short disproportionately reduces REM time.

Sleep Architecture

The structure and pattern of sleep stages throughout the night. A typical adult cycles through NREM and REM stages approximately every 90 minutes, completing 4–6 cycles per night. Early cycles are dominated by deep NREM sleep, while later cycles contain more REM sleep. Sleep architecture changes with age — older adults tend to spend less time in deep sleep and wake more frequently.

Sleep Debt

The cumulative effect of not getting enough sleep over time. If you need 8 hours but consistently get 6, you accumulate 2 hours of sleep debt each night. Small amounts of debt can be recovered with a few nights of longer sleep, but chronic sleep debt (weeks or months of insufficient sleep) is associated with impaired concentration, weakened immunity, and increased risk of cardiovascular disease. Weekend lie-ins only partially repay accumulated debt.

Sleep Efficiency

The percentage of time spent in bed that is actually spent sleeping. Calculated as (total sleep time ÷ time in bed) × 100. A healthy sleep efficiency is 85% or above. If you spend 8 hours in bed but only sleep for 6, your efficiency is 75%, which suggests issues with sleep onset, maintenance, or spending too long in bed. Sleep restriction therapy, a common CBT-I technique, improves efficiency by reducing time in bed to match actual sleep time.

Sleep Hygiene

The set of habits, behaviours, and environmental conditions that promote consistent, good quality sleep. Key principles include maintaining a regular sleep schedule, keeping the bedroom cool (16–18°C), dark, and quiet, avoiding caffeine after early afternoon, limiting screen use before bed, and having a wind-down routine. Good sleep hygiene is the foundation of every sleep improvement programme and is recommended by the NHS as a first step for anyone struggling with sleep.

Sleep Latency (Sleep Onset Latency)

The time it takes to fall asleep after getting into bed and turning off the light. A healthy sleep latency is between 10 and 20 minutes. Falling asleep in under 5 minutes is often a sign of significant sleep debt, not good sleep quality. Taking longer than 30 minutes regularly may indicate insomnia or poor sleep hygiene. Sleep latency is one of the key metrics used in clinical sleep assessments.

Social Jetlag

The misalignment between your biological clock and your social schedule. Most commonly seen when people sleep significantly later at weekends than on weekdays — effectively shifting time zones twice a week. A difference of 2 or more hours between weekday and weekend wake times is associated with poorer academic and work performance, higher BMI, and increased cardiovascular risk. The term was coined by chronobiologist Till Roenneberg.