The day-night cycle that occurs approximately every 24 hours is the:

A) biorhythm.
B) night-day cycle.
C) sleep cycle.
D) circadian rhythm.

Circadian Rhythm

The natural, internal process that regulates the sleep-wake cycle and repeats roughly every 24 hours, affecting physical, mental, and behavioral changes.

Biorhythm

A hypothetical cycle of physical, emotional, or intellectual fluctuations believed to affect human behavior.