Circadian rhythm

The (roughly) 24hr cycle of sleep and wakefulness found in all animals and some plants. Circadian rhythms are dictated by Melatonin concentrations in the body, and can be influenced by Zeitgebers (time givers), which are environmental cues for our body to help it understand the time of day. Zeitgebers include blue light hitting the eye, and the temperature of the environment; drops in both of which tell our bodies that evening is setting in, which triggers the release of melatonin, which creates a strong desire to sleep.