National Sleep Foundation

Glossary

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Selective Serotonin Reuptake Inhibitors (SSRIs) - A class of drugs typically used to treat depression and anxiety.

Sexsomnia (sleep sex) - A type of NREM Sleep Arousal Disorder that occurs when a person who is asleep engages in sexual acts (this differs from having an erotic dream).

Shift Work Disorder (SWD) - A Circadian Rhythm Disorder characterized by insomnia or excessive sleepiness resulting from a work schedule that interferes with the normal sleep period.

Short Sleepers - An individual who sleeps less than the normal amount, without impairment in daytime functioning; adult short sleepers habitually get 5-6 or fewer hours of sleep per 24 hours.

Sleep Cycles - Sleep is divided into 90-minute cycles of rapid eye movement (REM) and non-rapid eye movement (NREM) sleep, repeated three to six times during the night.

Sleep Diary - A log the patient keeps of how many hours slept each night, how many awakenings during the night and their durations, how long taken to fall asleep, how well rested the patient felt upon awakening, and how sleepy the patient felt during the day. Factors affecting sleep, such as the number of caffeinated or alcoholic drinks imbued, and nap or exercise times and lengths, may also be recorded.

Sleep-Disordered Breathing - Conditions during which respiration pauses occur during sleep due to the airways either complete or partial collapse of the person’s airways. These conditions include obstructive sleep apnea (OSA, in which no ventilation occurs) and hypopneas (reduced ventilation due to partial airway obstruction) lead to intermittent and abrupt reduction in blood oxygen levels. Symptoms include snorting, snoring, gasping, and choking in one’s sleep.

Sleep Enuresis (Bedwetting) - A parasomnia defined as recurrent, involuntary voiding of urine during sleep. Sleep enuresis is diagnosed as a disorder if it occurs chronically (i.e., at least twice per week) and does not respond to conventional therapies.

Sleep Habits - Practices that foster and maintain good sleep health. Good sleep habits include behaviors and conditions that can be consciously changed to improve an individual’s quality and quantity of sleep.

Sleep Latency - How fast a person falls asleep.

Sleep Paralysis - The paralysis experienced during sleeping that persists after awakening or just prior to falling asleep. Paralysis at sleep onset is often associated with narcolepsy; paralysis at wake onset is not.

Sleep-Related Breathing Disorders - Disorders caused by a reduction or suspension in breathing during sleep; these disorders are a primary cause of excessive daytime sleepiness. The most common types are Obstructive Sleep Apnea Syndrome (OSAS), Central Sleep Apnea Syndrome, and Sleep-Related Hypoventilation Syndromes (SRHS).

Sleep-Related Bruxism - A sleep disorder characterized by grinding and/or clenching the teeth during sleep, usually also associated with sleep arousals.

Sleep-Related Eating Disorder (SRED) - A type of NREM Sleep Arousal Disorder that involves recurrent episodes of involuntary eating and/or drinking during sleep, usually during partial arousals. Consumption of peculiar forms or combinations of food or substances is typical (e.g., cigarette and peanut butter sandwich).

Sleep-Related Groaning (catathrenia) - A parasomnia characterized by groaning or moaning sounds, which are usually clustered and occur within a long period of expiration (2-50 seconds) subsequent to a deep inspiration.

Sleep-Related Hallucinations - Realistic hallucinations that occur during sleep onset (hypnagogic) or upon awakening from sleep (hypnopompic), and that can be very frightening. They usually consist of complex visual images, although auditory, tactile, or kinetic events may also occur.

Sleep-Related Hypoventilation Syndromes (SRHS) - A type of sleep disorder characterized by abnormal ventilation and gas exchange that significantly worsens during sleep, or may only be present during sleep. The abnormalities result in increased blood carbon dioxide (CO2) levels (hypercapnea) and are often associated with low blood oxygen concentrations (hypoxemia).

Sleep-Related Leg Cramps - Painful sensations caused by sudden and intense muscle contractions, usually in the calf or the small muscles of the foot, which occur during sleep or wakefulness.

Sleep-Related Movement Disorders - Disorders diagnosed when an individual experiences insomnia, excessive daytime fatigue, or non-restorative sleep stemming from movements that occur either during sleep or near sleep onset.

Sleep-Related Rhythmic Movement Disorder (RMD) - A sleep-related movement disorder that involves large and stereotyped rhythmic movements, including head banging, head rolling, rocking on the elbows and knees, rocking and/or rolling the body from side to side.

Sleep Sex (sexsomnia) - A type of NREM Sleep Arousal Disorder that occurs when a person who is asleep engages in sexual acts (this differs from having an erotic dream).

Sleep Spindles - A sudden increase in wave frequency that occurs as a person moves from stage 1 to stage 2 sleep.

Sleep Starts (hypnic jerks) - Sudden, brief jerks that occur when a person is falling asleep, often associated with a subjective feeling of falling, a sensory flash, or a sleep-onset dream. Sleep starts are benign, but other, more complex motor behaviors may be associated with, or misinterpreted as, sleep starts that are symptomatic of a more serious condition (i.e., Restless Legs Syndrome).

Sleep Talking (somniloquy) - Talking that occurs during sleep, ranging from mumbling to uttering clear words and/or sentences.

Sleep Terrors - A type of NREM Sleep Arousal Disorder include occurs when a sleeping person appears to be awake, and experiences profound inconsolability and panic (often including hitting or running around).

Sleep-Walking - A type of NREM Sleep Arousal Disorder that occurs when a person sits up and/or gets out of bed and walks around while they are asleep.

Slow-Wave Sleep (SWS) - A stage of NREM sleep characterized by the presence of slow brain waves called “delta waves” interspersed with smaller, faster waves. Blood pressure falls, breathing slows, and temperatures drops even lower, with the body becoming immobile. Sleep is deeper, with no eye movement and decreased muscle activity, although muscles retain their ability to function.

Somniloquy (sleep talking) - Talking that occurs during sleep, ranging from mumbling to uttering clear words and/or sentences.

Stage 1 Sleep - A stage of Non-Rapid Eye Movement (NREM) sleep; a time of drowsiness or transition from being awake to falling asleep. Brain waves and muscle activity begin slowing down in this stage. People in stage 1 sleep may experience sudden muscle jerks, preceded by a falling sensation.

Stage 2 Sleep - A stage of Non-Rapid Eye Movement (NREM) sleep; a period of light sleep during which eye movements stop. Brain waves become slower, with occasional bursts of rapid waves (called sleep spindles) and spontaneous periods of muscle tone mixed with periods of muscle relaxation. The heart rate slows and body temperature decreases.

Stage 3 Sleep - A stage of Non-Rapid Eye Movement (NREM) sleep, also called “slow wave sleep” (SWS); characterized by the presence of slow brain waves called “delta waves” interspersed with smaller, faster waves. Blood pressure falls, breathing slows, and temperatures drops even lower, with the body becoming immobile.

Stimulus Control Therapy - A type of insomnia treatment that focuses on engaging in actions that reinforce a person’s association of bedtime and the bedroom with rapid sleep onset.

Suprachiasmatic Nucleus (SCN) - A group of cells located in the hypothalamus immediately above the optic chiasm, which receives input from the retina. The SCN generates a circadian wakefulness-promoting signal keeps people awake during the day and provides circadian rhythmicity to nearly every physiological system.