Which of the following is an example of an ultradian rhythm?

• A. Menstrual cycle
• B. Sleep cycle
• C. Seasons change
• D. Circadian rhythm

Answer: (B)

An ultradian rhythm refers to biological cycles that occur more than once in a 24-hour period. The sleep cycle is a prime example of this, as it typically consists of several stages of sleep that recur multiple times throughout the night, usually every 90 minutes. Each cycle consists of non-REM sleep, followed by REM sleep, and this pattern continues, creating multiple sleep cycles within a single night's sleep.

In contrast, the menstrual cycle is an example of an infradian rhythm, as it spans longer than 24 hours (approximately 28 days on average). Seasonal changes represent circannual rhythms, which occur over the span of a year. Circadian rhythms, on the other hand, are 24-hour cycles that include processes like the sleep-wake cycle but refer to biological changes that happen once a day. The defining characteristic of the sleep cycle as an ultradian rhythm is its occurrence multiple times within a single day.