Glossary
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- N1 (Stage 1) sleep
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The lightest stage of non-REM (NREM) sleep. Slow eye movements are often present.
- N2 (Stage 2) sleep
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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 (Stage 3) sleep
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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 one form of delta activity. The brain is less responsive to external stimuli, and it is difficult to awaken a person during slow-wave sleep. As such, this is considered the deepest stage of sleep. The 2007 revision of the sleep scoring manual reclassified what was previously Stages 3 and 4 sleep in the 1968 manual into a single class, Stage N3.
- Narcolepsy
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A rare sleep disorder marked by excessive sleepiness or sudden sleep attacks. A person with narcolepsy may immediately go from wakefulness to REM sleep. The disorder is also associated with cataplexy (see entry). An estimated 3 in 10,000 people are affected by narcolepsy.
- Neurotransmitter
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A chemical (such as serotonin or norepinephrine) that permits nerve signals to bridge the gap, or synapse, between nerve cells.
- Night terror
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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.
- Nightmare
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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.
- Nocturnal
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Occurring or active during the night rather than during the day.
- Non-declarative memory
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See procedural memory.
- Non-rapid-eye-movement (NREM) sleep
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The stages of sleep that do not include rapid-eye-movement patterns. According to current guidelines for sleep stage classification, NREM consists of three different stages N1, N2, and N3.
- Obstructive sleep apnea (OSA)
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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 most common in obese, middle-age to elderly males, but can also occur in children and females.
- Orexin (also referred to as hypocretin)
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A neurotransmitter that promotes wakefulness by stimulating brainstem arousal centers.
- Owl
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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.
- Parasomnias
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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
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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).
- Period
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A duration of time derived from the cyclical recurrence of a given phenomenon. In relation to circadian rhythms, period refers to the length of time for one cyclical occurrence of a circadian rhythm. In humans, the circadian pacemaker has a period of about 24.2 hours.
- Periodic limb movements disorder (PLMD)
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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)
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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
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A point or stage in a periodic process (such as a circadian rhythm) in relation to a given reference or starting point. Phase difference can be expressed in hours or in degrees (360 degrees corresponds to a full cycle). There will be a phase relationship between any two circadian rhythms (e.g., peak in circadian rhythm of alertness coincides with peak in circadian rhythm of temperature = no phase difference).
- Polysomnograph or polysomnogram
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A recording of a person’s sleep, using several physiologic signals such as the brain waves (electroencephaologram), eye movements (electroocculogram), muscle activity (electromyogram), as well as breathing, the amount of oxygen in the bloodstream, heart rate, etc. Polysomnograms are used to evaluate patients in a sleep laboratory for potential sleep disorders.
- Prefrontal cortex (PFC)
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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
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The long-term memory of skills and procedures, or “how-to” knowledge. Also called implicit memory.