Circadian rhythm
Your body's internal clock of about 24 hours. It largely sets when sleep comes and when you are awake.
A little more detail
An endogenous rhythm of about 24 hours, run by the suprachiasmatic nucleus in the hypothalamus and based on feedback loops of clock genes. Light is the strongest synchroniser; a shifted rhythm can lead to circadian rhythm sleep wake disorders.
Source: Patke A, Young MW, Axelrod S. Molecular mechanisms and physiological importance of circadian rhythms. Nat Rev Mol Cell Biol. 2020;21(2):67-84. doi:10.1038/s41580-019-0179-2