Physical, mental and behavioral changes that follow a 24-hour cycle are called Circadian rhythms. Affected by light and darkness.
They are found in: animals, plants and many tiny microbes.
The study of circadian rhythms
is called chronobiology.
  Circadian rhythms are controled by the suprachiasmatic nucleus.
The SCN contains about 20,000 nerve cells and is located in the hypothalamus.
Circadian rhythms influence sleep-wake cycles, hormone release,
body temperature and other bodily functions. They have been linked to various sleep disorders,
such as insomnia.  Abnormal circadian rhythms have also been associated with obesity, diabetes, depression, bipolar disorder and seasonal affective disorder.