amplitude: Magnitude, greatness of size.
anesthesia: Complete or partial loss of sensation, usually caused by artificially produced unconsciousness.
biological clock: A collection of cells that regulates an overt biological rhythm, such as the sleep/wake cycle, or some other aspect of biological timing, including reproductive cycles or hibernation.
cataplexy: Sudden muscle weakness associated with narcolepsy. It is often triggered by emotions such as anger, surprise, laughter, and exhilaration.
cerebral cortex: The brain’s outer layer of gray tissue that is responsible for higher nervous function.
circadian: Exhibiting a periodicity of 24 hours.
cyanobacteria: Blue-green algae.
cytokines: Molecules that regulate the functioning of the immune system.
cytoplasm: Protoplasm outside a cell nucleus.
delta waves: Brain waves with a frequency of 1 to 3 hertz that emanate from the forward portion of the brain during deep sleep in normal adults.
desynchronization: Lack of alignment between external signals and the biological clock.
diurnal: Active or occurring during the daytime; repeating once each 24 hours.
electroencephalogram (EEG): A measurement of the electrical activity associated with brain activity.
electromyogram (EMG): A measurement of the electrical activity associated with muscle movements.
electrooculogram (EOG): A measurement of the electrical activity associated with eye movements.
endocrine system: The ductless glands in the body that secrete hormones.
endogenous rhythms: Rhythms driven by an internal, self-sustaining biological clock rather than by signals that are external to the organism (for example, light).
endothermic animals: Animals that expend energy to maintain body temperature.
entrain: To reset or align with the biological clock.
exacerbate: To aggravate or increase the severity of.
exogenous rhythms: Rhythms that are directly regulated by an external influence, such as an environmental cue. They are not generated internally by the organism itself.
follicle stimulating hormone: A hormone produced in the pituitary gland that stimulates the growth of follicles in the ovary and induces spermatogenesis in the testes.
frequency: The number of times a periodic process occurs per unit of time.
hallucination: A false and distorted perception of objects or events.
homeostasis: The ability or tendency of an organism or cell to maintain internal equilibrium by adjusting its internal processes.
homeostatic regulation of sleep: Refers to the neurobiological signals mediating the pressure or urge to sleep.
hypnagogic hallucination: A “greater-than-life-like” dream experience that occurs during sleep. Hypnagogic hallucinations are sometimes associated with narcolepsy.
hypnogram: A graphical summary of the electrical activities occurring during a night’s sleep.
hypothalamus: The part of the brain that lies below the thalamus and regulates body temperature and metabolic processes.
insomnia: Sleeplessness; chronic difficulty with sleep onset or maintenance of sleep, or a perception of nonrefreshing sleep.
luteinizing hormone: A glycoprotein secreted by the pituitary gland. It stimulates the gonads to secrete sex steroids.
melatonin: A hormone secreted by the pineal gland that is derived from the amino acid tryptophan, which helps synchronize biological clock neurons in the suprachiasmatic nucleus.
narcolepsy: A chronic sleep disorder characterized by excessive and overwhelming daytime sleepiness (even after adequate nighttime sleep).
neurotransmitter: A chemical produced by neurons that carries messages to other neurons.
nocturnal: Relating to or taking place at night.
non–rapid eye movement (NREM) sleep: The early phase of sleep with no rapid eye movement.
obstructive sleep apnea (OSA) (sleep apnea syndrome, sleep-disordered breathing): A disorder in which breathing is frequently interrupted for brief intervals during sleep, resulting in intermittent decreases in blood oxygen levels and transient arousals from sleep, leading to poor sleep quality and excessive daytime sleepiness.
oscillation: The state or act of swinging back and forth with a regular, uninterrupted pattern.
parasomnias: Sleep disorders that include sleepwalking, sleep talking, and sleep terrors.
photoperiod: The light/dark or day/night cycle.
photoreceptor: A molecule or structure that can detect light.
pons: The brainstem region critical for initiating REM sleep.
rapid eye movement (REM) sleep: Deep sleep with rapid eye movements in which dreaming takes place.
restless legs syndrome: A neurologic movement disorder that is often associated with a sleep complaint.
seasonal affective disorder (SAD): A form of depression caused by inadequate bright light reaching the biological clock in the suprachiasmatic nucleus. Consequently, treatment often involves the use of light therapy.
sleep hygiene: The collection of behaviors and environmental conditions that influence the length and quality of sleep.
sleep paralysis: The temporary inability to talk or move when falling asleep or waking up. It occurs normally during REM sleep.
suprachiasmatic nucleus (SCN): The part of the brain (in the hypothalamus) that contains the biological clock.
thalamus: The area of the brain that relays sensory information to the cerebral cortex.
thermoregulation: Maintenance of internal body temperature regardless of environmental temperature.
ubiquitous: Seeming to be everywhere.
ultradian rhythm: A perodicity of less than 24 hours.
unihemispheric sleep: A type of sleep in which one side of the brain is asleep while the other is awake. This phenomenon is observed most notably in birds (like those that make long, transoceanic flights) and aquatic mammals (like dolphins and porpoises).