A biological rhythm are recurring cycles that occur within living organisms. And circadian rhythms is a special class of biological rhythms with period of 24-hours.

The circadian system primarily regulates alertness, influencing when we feel sleepy or awake throughout the day.

However, the natural human circadian cycle tends to be slightly longer than 24 hours. Daily exposure to sunlight, the primary external factor known as a zeitgeber (German for “time-giver”), helps reset and align our internal clock to the Earth’s 24-hour day-night cycle. Without this regular adjustment, our circadian rhythm would drift out of sync with the environment.

Circadian rhythm can be measured by melatonin levels and core body temperature.