tiny gland located at the top of the brain between the cerebral hemispheres and the cerebellum; comprises specialized neural cells that function as photoreceptors that detect the daily oscillation between sunlight and darkness; functions with the hypothalamus as an oscillator to keep the body’s internal daily (circadian) rhythms of activity and rest; secretes melatonin into the bloodstream during nighttime hours, communicating the daily changes in the length of daylight (the photoperiod) to keep the body’s endogenous circadian rhythms in synchrony with the Earth’s daily rhythms; see Photoperiodism and Reproduction Timing