A graphical representation of the cycles of activity and rest, usually indicating wakefulness and sleep. Recorded activity levels are used to assign periods of sleep or wakefulness, which are plotted over many days and nights. Each 24-hour period is plotted below the prior period. Thus, the periods of estimated sleep and wakefulness would align in people who have regular sleep/wake schedules. 


The use of recordings of body motion to display activity patterns across many consecutive days and nights. These temporal patterns are used for classifying periods of relative rest that usually indicate sleep, and periods of relative activity that usually indicate wakefulness. Actigraphy can be conducted on people in their usual setting (for instance, at home) by wearing a wristwatch-sized actimeter device that measures and records accelerations of the wrist. Thus, overall sleep and wakefulness patterns can be estimated over many days and nights without the need for directly recording sleep and wakefulness by use of a polysomnogram in a laboratory setting (see entry). 


An essential molecule found in all cells and involved in providing the energy needed for many biochemical processes. Adenosine also appears to play an important role in sleep initiation. The concentration of adenosine surrounding cells in some of the arousal centers of the brain increases with prolonged wakefulness and inhibits arousal. For that reason, adenosine is thought to be involved in the initiation of sleep. In contrast, use of caffeine promotes wakefulness by blocking the actions of adenosine. 

Advanced sleep phase syndrome (ASPS)

A daily sleep/wake rhythm in which the onset of sleep and the time of awakening are earlier than desired. A person with ASPS wakes up earlier and wants to go sleep earlier than most individuals. 

Alpha activity

A pattern of brainwave activity detected by electroencephalography with a rhythm at 8–13 Hz or cycles per second. The presence of alpha activity usually indicates relaxed wakefulness with eyes closed, which often precedes the onset of sleep. 


The height of a wave. The amplitude of brainwaves derived from electroencephalography (EEG) changes with depth of sleep. For EEGs, amplitude is measured in voltage, typically microvolts. 


Drugs that block the action of histamine, lessening the effect of allergic reactions. Antihistamines have a sedative side effect and are often found in many over-the-counter sleep medications. 


A cessation of breathing. When apnea occurs during sleep due to an obstruction of the airways despite efforts to breathe—akin to an extreme form of snoring—it is called “obstructive sleep apnea.” If apnea occurs because there is no attempt to breath during sleep, this is called “central sleep apnea.” 

Autonomic nervous system

The part of the nervous system that controls many of the involuntary body functions such as blood pressure, heart rate, and breathing, including the dilation or constriction of arteries and small airways in the lungs. Subdivisions of the autonomic nervous system are the sympathetic and parasympathetic nervous systems. These 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 increasing parasympathetic activity (increased braking). 


A class of medications used to relieve nervousness, tension, and other psychological symptoms. Benzodiazepines attach to specific receptors in the brain and affect levels of neurotransmitters. Medications using benzodiazepines are frequently used as sleep aids. 

Beta activity

A pattern of brainwave activity detected by electroencephalography with a rhythm at 13–35 Hz or cycles per second. The presence of beta activity usually indicates alert wakefulness or vigilance. 

Body mass index (BMI)

An estimate of an individual’s relative body fat calculated from his or her height and weight. It is calculated using the formula BMI=weight (kg)/height (m)2. Obesity is medically defined as a BMI over 30. 

Brain plasticity or neuroplasticity

Changes that occur in the brain through the reorganization of neuronal connections and receptor density. Sleep may provide one of the best circumstances for this neural restructuring to take place. 


The brain structure that is the major communication route between the brain and spinal cord. The brain stem contains many of the arousal centers responsible for maintaining wakefulness; it also contains other connections in specific areas that control heart rate, breathing, and other vital functions. 


Patterns of electrical activity in the brain. These patterns can be recorded from scalp recordings using electroencephalography (EEG) and are described based on frequency, amplitude, and shape characteristics. See also EEG. 


A chemical compound with stimulant properties. Caffeine affects the central nervous system by binding to adenosine receptors in the brain, thereby inhibiting the sleep-promoting actions of adenosine that normally increase with prolonged wakefulness. Caffeine is the most widely used substance to increase vigilance and extend the time spent awake. 


Sudden paralysis of some or nearly all skeletal muscles brought on by strong emotions such as those that accompany heartfelt laughter and anger. Cataplexy is a hallmark of narcolepsy. 

Central sleep apnea

A cessation of breathing attempts for at least 10 seconds during sleep. Central sleep apnea is caused when respiratory control centers in the brain temporarily pause their activation of the breathing muscles. This occurs most often during sleep when the carbon dioxide levels in the blood are reduced below normal, and can be triggered by prior overbreathing. Central sleep apnea can also occur with damage to certain neural pathways involved in respiratory control. See also obstructive sleep apnea (OSA). 

Cerebral cortex

The brain’s outer layer of gray matter surrounding the cerebrum. The cerebral cortex carries out most aspects of “higher” or “executive” brain function such as planning, analysis, thought, and memory. The cortex also contains many of the sensory areas as well as areas involved in the initiation of voluntary movement. 


The study of the way time interacts with biological systems. One of the most pronounced chronobiological systems studied is the internal circadian system, which organizes our physiology and behavior over a 24-hour period. 


A term derived from the Latin words circa and dies, meaning “about a day.” An internal circadian pacemaker in the hypothalamus of the brain organizes our physiology and behavior over a 24-hour period, including the sleep/wake cycle. 

Circadian alerting system

An internal circadian pacemaker-dependent signal that promotes wakefulness and thereby inhibits sleep. As such, the circadian alerting system is one of the main processes that regulates sleep behavior in humans. 

Circadian clock

See Suprachiasmatic nucleus (SCN) [also referred to as circadian clock, circadian pacemaker, or internal biological clock]

Circadian pacemaker

See Suprachiasmatic nucleus (SCN) [also referred to as circadian clock, circadian pacemaker, or internal biological clock]

Circadian rhythm

A day/night pattern in many physiological and behavioral variables occurring over a 24 hour period, generated internally by a circadian pacemaker, and persisting under constant environmental conditions. 

Circadian rhythm sleep disorder

A condition in which a person’s sleep/wake schedule is out of synchrony with, or occurs at an unusual phase of, the internal circadian clock. Circadian rhythm sleep disorder can occur, for instance, with shift work, jet lag, advanced sleep-phase syndrome, and delayed sleep-phase syndrome. This mismatch can lead to insomnia during attempted sleep times and excessive sleepiness throughout scheduled wake times. 

Cognitive behavioral therapy (CBT)

A form of therapy that aims to correct ingrained patterns of negative thoughts and behaviors. CBT has been shown to be effective in treating some kinds of insomnia. 

Cognitive function

Brain mechanisms involved with thinking, reasoning, learning, and remembering. Getting healthy sleep improves cognitive function. 

Cognitive impairment

Problems in mental functions, including intelligence, judgment, learning, memory, speech, and thinking. Cognitive impairment that affects judgment, coordination, and/or the ability to process new information quickly can increase the risk of accidents. One potential outcome of insufficient sleep is cognitive impairment. 

Continuous positive airway pressure (CPAP)

A treatment for obstructive sleep apnea in which a continuous stream of air under pressure is delivered through a mask worn over the nose, or nose and mouth, to keep the sleeper’s airway open. 


A steroid hormone released by the outer part (cortex) of the adrenal glands. Cortisol has a robust circadian rhythm under constant conditions and is also released in stressful situations. The circadian-related increase in cortisol occurs usually before waking up and is thought to be a way of preparing the body for the stresses of the waking day. Cortisol release may inhibit sleep; this is one reason that stressful activities before bedtime are not recommended. 

Declarative memory

Memory for facts (semantic memory) and for events (episodic memory); also called explicit memory. 

Delayed sleep phase syndrome (DSPS)

A daily sleep/wake rhythm in which the onset of sleep and the time of awakening are later than desired. A person with DSPS wakes up later and tends to go sleep later than most individuals. 

Delta activity

EEG activity of 75-microvolt amplitude (peak-to-peak) and frequency less than 4 Hz or cycles per second. 


Occurring or active during the daytime rather than at night. 


A series of thoughts, images, or emotions occurring during sleep and especially during REM sleep. Most dreams are not remembered, unless people wake up and immediately recall the dream. 

Electroencephalogram (EEG) or electroencephalograph

A recording of brain waves obtained by attaching flat metal discs (electrodes) to the scalp. EEGs reflect the summation of the activity of millions of neurons that are close to the electrode. They show characteristic changes in brain wave amplitude and frequency during sleep and wakefulness. 


Arising from within a system. For example, circadian rhythms are endogenous�they persist even when the environment and behaviors are kept constant, and occur because of influences from the circadian pacemaker in the brain. 


The synchronization between two rhythms. Entrainment occurs when the internal circadian system becomes in synchrony with an external system, such as the daily light/dark cycle. This entrainment is caused by the effects of light on the internal circadian pacemaker. 

Excessive daytime sleepiness (EDS)

A difficulty in staying awake during the daytime when a person intends to be alert. EDS may be marked by lapses into sleep during sedentary moments during the day. 


Resulting from an external factor or change. For example, environmental stimuli such as light are exogenous. 

Free running

A term that describes the circadian rhythm that is not subject to any kind of entrainment (see entry). For instance, experimental subjects in a laboratory under constant environmental conditions, including only dim light, will have circadian rhythms that “free run” at their inherent rate, normally slightly longer than 24 hours. In contrast, the circadian rhythms of people living in a normal environment with a day/night light/dark cycle are reset or entrained each day by light.


A hormone produced in the stomach that stimulates appetite. The body’s production of ghrelin may be influenced by a person�s amount of sleep. See also leptin. 

Growth hormone

A hormone secreted by the pituitary gland that promotes growth of the body and influences the metabolism of proteins, carbohydrates, and lipids. The secretion of growth hormone peaks shortly after sleep onset. 

Hertz (Hz)

The number of cycles per second; the unit of measurement applied to a rhythmic event, such as brainwaves. 


A part of the brain that plays a central role in many functions, including processing of memories. 


A substance released by cells that causes symptoms of an immediate allergic reaction. Histamine released in certain areas of the brain promotes arousal. Conversely, anti-histamine medications can promote sleep. 


Maintenance of the body’s internal environment, for instance, through regulation of blood pressure, temperature, water, and nutrient content. This stability in the long term is usually achieved by a control mechanism called “negative feedback,” where short-term changes in a regulated physiological variable are counteracted by reflexes that bring the physiological variable back to a stable set-point level. 

Homeostatic sleep drive

The drive to sleep that accumulates during prolonged wakefulness, and lessens during sleep. Sleep homeostasis is one of the primary modulators of sleep in humans. 


Occurring in the time period surrounding sleep onset. 

Hypnagogic hallucinations

Vivid images, sounds, or tactile feelings that occur when drifting off to sleep. The hallucinations can be frightening and are sometimes accompanied by sleep paralysis. 


A graph that summarizes the pattern of sleep stages across a night, for instance, as recorded in the sleep laboratory. 

Hypnopompic hallucinations

Vivid images, sounds, or tactile feelings that occur when waking from sleep. The hallucinations can be frightening and are sometimes accompanied by sleep paralysis. 


In sleep medicine, hypnotic refers to an agent that promotes sleep. 


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 hypocretins. Hypocretins are also known as orexins. 


A deep brain region just in front of the brainstem that regulates arousal, sleep, hunger, body temperature, and other fundamental behaviors. The neurons that produce hypocretins lie in the hypothalamus. 


Trouble falling asleep, staying asleep, or waking up too early with resultant daytime feelings of lack of restorative sleep. 


A hormone produced by the pancreas that directs the passage of glucose, amino acids, and fatty acids into cells and promotes their storage. 

Insulin resistance

A condition in which the body does not respond normally to insulin. Insulin resistance can lead to type 2 diabetes and heart disease. Current research is exploring potential links between inadequate sleep and elevated risk of insulin resistance and diabetes. 

Jet lag

A condition caused by air travel through changing time zones and the resulting desynchrony of the body clock and external time. Jet lag is marked by fatigue, insomnia, nausea, and irritability.

K complex

A sharp, negative EEG wave followed by a high-voltage slow wave, lasting at least 0.5 second. K complex waves are one of the characteristics of N2 (Stage 2) sleep. 


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


A hormone that stimulates satiety, and thus decreases appetite. Reduced sleep duration may lead to a reduction in leptin, thereby stimulating appetite. See also ghrelin. 

Maintenance of Wakefulness Test (MWT)

A sleep laboratory test that consists of several 40-minute periods of rest during which a patient must remain awake while sitting. This test is often used to determine a patient’s ability to stay awake, even in boring circumstances. 

Memory acquisition: The initial step in forming a memory

The mechanism by which newly perceived information is received in the brain. 

Memory consolidation: The second stage of memory

The neural processing that occurs after information is initially registered, which contributes to its permanent storage in memory. 

Memory recall: The final stage of memory

The ability to access and utilize stored information. 


A very brief period of sleep, lasting only a few seconds. Microsleeps occur most frequently when a very sleepy person is trying to stay awake�for example, when driving while sleepy. 


A central nervous system stimulant that promotes wakefulness. Modafinil is primarily used to treat the symptoms of excessive daytime sleepiness (EDS) in people with narcolepsy. See also excessive daytime sleepiness (EDS) and narcolepsy. 

Motor neuron

A neuron that conveys signals from the central nervous system to a muscle. 

Multiple Sleep Latency Test (MSLT)

A sleep laboratory test that consists of five 20-minute nap opportunities spread across the day. This test is used to measure a patient’s sleepiness. People with narcolepsy usually fall asleep in less than 8 minutes and have REM sleep in two or more of these naps.

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 REMsleep. 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. 

Quiet sleep

An alternate term for non-REM sleep. In the quiet phase of sleep most physiological activities are very stable and regular, and many are reduced compared to wakefulness. 

Rapid-eye-movement (REM) sleep

A resting state with little consciousness of the environment, high cortical activity, vivid dreams, and periods of rapid eye movements. Nearly all skeletal muscles are paralyzed during this state, which prevents an individual from acting out dreams. Also called paradoxical or dreaming sleep. 

Restless legs syndrome (RLS)

A condition characterized by achy or unpleasant feelings in the legs associated with a need to move. Most prominent at night, making it hard to fall asleep or stay asleep. 

Seasonal affective disorder (SAD)

Sadness and depression that is brought on by a lack of exposure to sunlight. SAD usually appears in the fall or winter and subsides in the spring. 

Shift work

A general term to describe a job that requires an individual to work other than the standard working hours of mid-morning to late afternoon, Monday through Friday. For instance, shift work may involve working from midnight until 7:00 a.m. 

Shift work sleep disorder

A chronic condition that is directly related to a shift work schedule. Two primary symptoms are insomnia during times when a person is trying to sleep and excessive sleepiness when a person needs to be awake and alert. 


A rest or short nap, usually after the midday meal. 

Sleep apnea

A cessation of breathing during sleep. In adults episodes last at least 10 seconds; in children they last at least the duration of two breath cycles. See also central sleep apnea and obstructive sleep apnea. 

Sleep debt

An individual’s accumulated sleep loss from insufficient sleep, regardless of cause. 

Sleep drive

See homeostatic sleep drive. 

Sleep efficiency

The proportion of sleep in the period available for sleep; that is, the ratio of total sleep time to time in bed. 

Sleep inertia

The grogginess that occurs immediately after awakening, which can adversely affect cognitive and psychomotor abilities. Overcoming sleep inertia can take minutes or even an hour or more. This depends on multiple factors, possibly including the stage of sleep from which a person awakes and the duration of prior sleep. 

Sleep latency

How long it takes a person to fall asleep from the onset of the potential sleep period; for instance, time measured from turning the lights out to the onset of sleep. 

Sleep onset

The transition from waking to sleep, normally into non-REM N1 sleep. 

Sleep paralysis

Brief episodes of immobility when falling asleep or upon awakening. The paralysis typically affects all muscles except those needed for breathing. This is probably the occurrence during wakefulness of the paralysis that normally occurs during REM sleep. 

Sleep spindle

Spindle-shaped burst waves on the EEG occurring at 12–14 Hz for over half a second. Sleep spindles are one of the identifying features of N2 sleep. 

Sleep-wake homeostat

See Homeostatic sleep drive

Slow-wave sleep (SWS)

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

Suprachiasmatic nucleus (SCN) [also referred to as circadian clock, circadian pacemaker, or internal biological clock]

The internal circadian pacemaker is a small group of nerve cells located in the hypothalamus that controls the circadian cycles and influences many physiological and behavioral rhythms occurring over a 24-hour period, including the sleep/wake cycle. 

Sympathetic nervous system

One of two subdivisions of the autonomic nervous system; it intensifies certain body activities and releases stress hormones in response to perceived or real dangers. These 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).


It is the small space between two nerve cells, or between a nerve cell and a muscle or gland through which connecting electrical impulses are transmitted.


The act of maintaining an organism’s temperature within certain normal boundaries. This can be achieved, for instance, by shivering to create heat or sweating to lose heat. 

Theta activity

EEG activity with a frequency of 4–7 Hz or cycles per second. 

Tuberomammillary nucleus (TMN)

An area of the hypothalamus that contains neurons that release histamine. This neurotransmitter stimulates arousal. 

Ventrolateral preoptic area (VLPO)

An area of the brain in the hypothalamus that plays a key role in promoting sleep by inhibiting the brain’s primary arousal centers, such as the Tuberomammillary Nucleus (TMN).