Ultradian rhythms are the most frequent and occur more than once in 24 hours.
- Going through the 5 stages of sleep several times each night.
Circadian rhythms occur about ("circ") once a day ("dia").
Earth turns in a circle = day.
- Sleep-wake cycle.
- Daily fluctuation in body temperature.
Infradian are the most "infrequent". They occur less than once a day.
- Menstrual cycle.
- Season affective disorder (SAD - yearly cycle of depression starting in the winter).