Pineal Gland: A small endocrine gland in the vertebrate brain which produces melatonin.
Circadian Rhythms: Physical, mental, and behavioral changes that follow a daily cycle; they respond primarily to light and darkness in an organism's environment.
Endocrine System: The collection of glands that produce hormones that regulate metabolism, growth and development, tissue function, sexual function, reproduction, sleep, and mood among other things.