Cyclical variations in physiological and biochemical function, level of sleep-wake activity, and emotional state.
Circadian rhythms have a cycle of about 24 hours, ultradian rhythms have a cycle that is shorter than 1 day, and infradian rhythms have a cycle that may last weeks or months.

American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders. 5th ed. Washington, DC. American Psychiatric Association. 2013. Internet. Accessed on June 21, 2017.