Chronobiology

upright=1.8|thumb|Overview, including some Physiology|physiological parameters, of the human circadian rhythm ("biological clock")

Phenology is the study of periodic events in biological life cycles and how these are influenced by seasonal and interannual variations in climate, as well as habitat factors (such as elevation).

thumb|right|Owl|Owls are well known for being nocturnal, but some owls are active during the day.

biologically inherent cyclic variation or recurrence of an event or state, such as the sleep cycle, circadian rhythms, or periodic diseases

purported correlation between the lunar cycle and behavior and physiological changes in living beings on Earth, including humans

biological rhythm with period that is longer than 24 hours

oscillation between the slow-wave and REM phases of sleep

specific actions or reactions of an organism that recur with a regularity more frequent than once in 24 hours