It's an innate periodic cycle in an organism's physiological processes as in sleep & wake cycles
The cycle time between successive peaks (or troughs) is called the 
period of the cycle and ... organisms, is strong evidence that variations in the 
activity of physiological ... CIRCADIAN OSCILLATION OF SLEEP AND WAKING 
STATES [DOC] [TLE] Daynight cycles and the sleep-promoting factor, Sleepless, affect ... [PAR] Feb 25, 2014 ... We also examine whether behavioral sleepwake cycles, which are ... habitat 
and have influenced the evolution of life since its origin (1).