A rhythm of activity and inactivity lasting about one day defines a(n)

A) sleep spindle.
B) altered state of consciousness.
C) sleep cycle.
D) circadian rhythm.

Circadian Rhythm

A 24-hour internal clock cycle that regulates physiological and behavioral processes in living organisms.

Sleep Cycle

A progression through various stages of NREM and REM sleep that occurs several times during a typical night, important for restorative processes.

Sleep Spindle

A burst of oscillatory brain activity visible on an EEG that occurs during stage 2 of sleep.