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N1 (Stage 1) sleep

The lightest stage of non-REM (NREM) sleep. Slow eye movements are often present.

N2 (Stage 2) sleep

The second stage of non-REM (NREM) sleep. EEGs during N2 sleep show sleep spindles and K complexes on a background of relatively low-voltage, mixed-frequency EEG activity.

N3 sleep (Stage 3 or slow-wave sleep [SWS])

The deepest stage of non-REM (NREM) sleep, characterized by a larger amount of synchronized slow-wave EEG (brainwave activity) than in other stages. These slow waves are called delta activity. During slow-wave sleep the brain becomes less responsive to external stimuli; it is considered the deepest sleep as it is the hardest stage from which to awaken. The 1968 categorization of the combined Sleep Stages 3 or 4 was reclassified in 2007 as Stage N3.


A sleep disorder marked by excessive daytime sleepiness and sometimes cataplexy and other symptoms. Narcolepsy affects an estimated 1 in 2,000 people.


A nerve cell in the brain and other parts of the nervous system that conveys nerve signals through complex networks regulating sleep, arousal, consciousness, speech, walking, and many other behaviors.


A chemical (such as serotonin or norepinephrine) that permits nerve signals to bridge the gap, or synapse, between nerve cells. Neurotransmitters usually excite or inhibit activity in downstream, target neurons.

Night terror

A sleep disturbance that occurs during the non-dreaming stages of sleep (generally, deep slow-wave sleep). An individual experiencing a night terror is often overcome by panic but cannot easily be aroused because of the deep level of sleep in which this phenomena occurs. Unlike nightmares, night terrors generally lack visual imagery and are most often not remembered upon awakening. See also nightmares.


A sleep disturbance that occurs during REM sleep (dreaming sleep) and has frightening content. Nightmares often result in either partial or total arousal from sleep since it is relatively easy to wake from REM sleep. See also night terror.


Occurring or active during the night rather than during the day.

Non-declarative memory

See procedural memory.

Non-rapid-eye-movement (NREM) sleep

A resting state in which a person has little consciousness of the environment, low cortical activity, and almost no internal thoughts. NREM sleep consists of three different stages: N1 (light sleep), N2, and N3 (deep sleep).

Obstructive sleep apnea (OSA)

A condition in which a person stops breathing for at least 10 seconds, with such cessations occurring repeatedly during sleep. Obstructive sleep apnea is akin to an extreme form of snoring. Sleep apnea is usually due to complete or partial obstruction of the airway in the back of the throat. Sleep apnea is common in obese, elderly males, but can also occur in children and females.


Neurotransmitters that normally promote stable wakefulness and help regulate REM sleep. In people who have narcolepsy with cataplexy, there is a loss of neurons that produce orexins. Orexins are also known as hypocretins.


A term describing individuals who are predisposed to stay up late and to be at their best in the evening. Also termed “evening-type.” Contrasts with “lark” (see entry). Such owl and lark predispositions may have a genetic component.


Troubling or undesirable behaviors that intrude during sleep, such as sleepwalking. They often are associated with abnormal or partial arousal and typically disrupt normal sleep.

Parasympathetic nervous system

One of two subdivisions of the autonomic nervous system. The parasympathetic nervous system branch generally is involved in recuperative and maintenance functions of the body, such as digestion. These two subdivisions usually work in opposition (like a brake and an accelerator in a car). Thus, for example, slowing the heart can be achieved by simultaneously decreasing sympathetic activity (reduced acceleration) and by increasing parasympathetic activity (increased braking).

Periodic limb movements disorder (PLMD)

A sleep disorder characterized by leg movements or jerks that typically occur every 20 to 40 seconds during sleep, causing sleep to be disrupted and leaving the person with excessive daytime sleepiness.

Periodic limb movements of sleep (PLMS)

Leg movements or jerks that typically occur every 20 to 40 seconds during sleep. The term periodic limb movement disorder is used if the leg movements produce daytime symptoms (see entry).

Phase shift

A shift that moves one’s typical sleep or wake time to a different part of the circadian cycle.

Polysomnogram or polysomnograph

A recording of a person’s sleep, using several physiologic signals such as brain waves (electroencephalogram), eye movements (electrooculogram), and muscle activity (electromyogram), as well as breathing patterns the amount of oxygen in the bloodstream, heart rate, and body position. A polysomnogram is used to evaluate patients in a sleep laboratory for potential sleep disorders.

Prefrontal cortex (PFC)

A region of the brain that plays a critical role in the formation of cognition, directing goal-oriented thoughts, and executive function. The PFC is particularly vulnerable to the effects of sleep deprivation.

Procedural memory

The long-term memory of skills and procedures, or “how-to” knowledge. Also called implicit memory.

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