Inherent rhythm that controls or initiates various biological processes: linked to the cycle of days, seasons, etc. Biological clocks: endogenous rhythms (control comes from within, neural system that times behaviour, allows animals to anticipate events before they happen, not simply driven by external cues from the environment. Free-running rhythms: body"s rhythm in the absence of external cues, without input from external cue, rhythms within periods of 24. 1-24. 3 hours. Zeitgebers: environmental event that entrains biological rhythms, a time giver programs the clock, example: light resets the biological clock, entrainment- determination or modification of the period of a biorhythm. Jet lag- disruption in the entrainment of a person"s biological clock: east west =easier to adjust, west east = harder to adjust. Main neural clock: suprachiasmatic nucleus (scn, main pacemaker of circadian rhythms located just above the optic chiasm, x crossing over of eye nerves. Melatonin: hormone secreted by the pineal gland, released during the dark phase of the day-night cycle.