Our circadian rhythm is the rhythm that is generated and controlled from within us. It is the biological clock that resides in our brains. It is endogenous. Circadian rhythms help us regulate our sleep and wake cycle. In addition, it regulates our eating and drinking habits, body temperature, and hormone release. The human circadian rhythm is approximately 24 hours AND 10 minutes. All living species (including bacteria surviving more than 24 hours) function on a  circadian rhythm.

Kleitman, N. (1963). Sleep and wakefulness. Chicago: University Press.