A-C | D-F | G-J | K-M | N-P | Q-S | T-Z


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.

back to top

A-C | D-F | G-J | K-M | N-P | Q-S | T-Z