a. Circadian rhythm: (circa=about; dies=day) - Approximately 24-hour cycles that are generated endogenously by an organism (eg, sleep-wake cycle)

b. Ultradian rhythm: Biological rhythms that are shorter than a 24-hour cycle (eg, sleep stages)

c. Entrainment: Synchronization of the circadian rhythms with environmental cues

d. Free-running clock: Persistence of circadian rhythms in the absence of environmental cues (eg in patients with impaired ability to entrain or in those without time cues)

e. Zeitgeber (time giver): Specific environmental variables that provide time cues (eg, light, food-availability cycles)

f. Sleeplessness (insomnia): Difficulty initiating or maintaining sleep

g. Circadian rhythm disorders: Malfunction of the circadian timing system or the biological clock

h. Cortisol: A corticosteroid hormone produced by the adrenal cortex. It is involved in the response to stress; it increases blood pressure, blood sugar levels, may cause infertility in women, and suppresses the immune system.

i. Serotonin (5-hydroxytryptamine, or 5-HT): is a monoamine neurotransmitter. It is believed to play an important role in the regulation of mood, sleep, emesis (vomiting), sexuality and appetite