Circadian rhythm: A daily cycle of biological activity of approximately 24 hours. It is driven by an internal clock and influenced by regular variations in the environment, such as the alternation of night and day.

Hormone: A chemical compound that is produced in a gland and then carried to another part of the body by the blood. Hormones control many important body activities, such as growth. Hormones act by causing and adjusting chemical reactions in the body.

Sleep: A natural state of rest that occurs at regular times. During sleep, growth is thought to take place, and energy is conserved and stored away. Dreams take place during a stage of sleep called REM.