Circadian rhythm

The state of being alert (beta waves) or relaxed (alpha waves) before falling asleep.

Dreams

The lightest phase of sleep where a person is easily awoken. Brain activity shifts to theta waves

Electroencephalograph (EEG)

A period of light sleep where heart rate and temperature drop. The EEG shows distinct "sleep spindles" and "K-complexes".

Hypersomnia

Excessive daytime sleepiness despite getting enough nighttime sleep.

Insomnia

Persistent difficulty falling or staying asleep.

Melatonin

A hormone produced in response to darkness that helps signal the body it is time to sleep.

Napping

Short periods of sleep during the day, which may or may not relieve sleepiness depending on the underlying condition.

Narcolepsy

A neurological disorder causing sudden "sleep attacks" and a disordered regulation of REM sleep.

Night Terrors

Night terrors occur in NREM sleep and involve intense fear without a clear dream, while nightmares are scary dreams occurring in REM sleep.

Nightmares

Night terrors occur in NREM sleep and involve intense fear without a clear dream, while nightmares are scary dreams occurring in REM sleep.

Parasomnia

disruptive sleep disorders involving undesirable physical behaviors or experiences—such as sleepwalking, sleep terrors, or nightmares—occurring during sleep, upon waking, or between sleep stages.

Restless Legs Syndrome

a common neurological disorder characterized by an irresistible urge to move the legs, typically triggered by uncomfortable sensations like crawling or tingling when resting or at night.

Sleep

a vital, active process necessary for cognitive function, memory consolidation, and physical restoration.

Sleep apnea

a common, serious disorder where breathing repeatedly stops and starts during sleep, preventing restful sleep and lowering blood oxygen levels.

Sleep deprivation

: The condition of not getting enough sleep, leading to cognitive impairment and health risks.

Sleep paralysis

The brain "paralyzes" voluntary muscles (muscle atonia) to prevent acting out dreams.

Sleep study

(polysomnography) is the primary clinical tool used to diagnose these conditions by recording brain waves, oxygen levels, and movements overnight.

Snoring

the sound caused by air forcing its way through blocked or narrowed airways, making throat tissues vibrate during sleep

Stage 1 (Non-REM) sleep

the brief, lightest transitional phase between wakefulness and sleep, typically lasting 1–7 minutes and making up 2-5% of total sleep.

Stage 2 (Non-REM) sleep

a period of light sleep, often lasting 10–25 minutes, that constitutes about 50% of total adult sleep time.

Stage 3 (Non-REM) sleep

Also known as slow-wave sleep or deep sleep, characterized by slow delta waves. This stage is critical for tissue repair and growth.

Stage 4 (REM Sleep)

high brain activity, vivid dreaming, rapid eye movements, and temporary muscle paralysis.

Somnambulism

a disorder where complex behaviors (like walking) occur during deep non-REM (N3) sleep, typically in the first third of the night

Wakefulness

a state of active alertness and consciousness, distinct from sleep, driven by complex neural networks involving neurotransmitters like norepinephrine, serotonin, and orexin.