Outline the different types of biological rhythms.

There are three main types of internal rhythms that influence human biology: ultradian, circadian and infradian. The duration and nature of these rhythms vary with individual differences and are affected by endogenous (internal - eg. hormones, body temperature) and exogenous (external - eg. time, weather, social situation) factors.
Ultradian cycles have a duration of less than 24 hours, an example being the 90 minute sleep cycle, which controls our sleeping state in each of the five stages. The circadian rhythm determines the sleep-wake cycle, which repeats approximately every 24-26 hours in humans (Siffre). Infradian cycles are those that repeat less regularly than every 24 hours, such as the menstrual cycle, which has a duration of approximately 28 days.