Final answer:
Sleep cycles are considered ultradian rhythms because they occur multiple times within a 24-hour period, unlike circadian rhythms that occur once per day, with each sleep cycle lasting around 90 to 110 minutes.
Step-by-step explanation:
Sleep cycles are considered ultradian rhythms because they occur more frequently than once per day. Unlike circadian rhythms, which follow a roughly 24-hour cycle aligned with the Earth's rotation, ultradian rhythms such as the stages of sleep recur throughout the 24-hour period. During sleep, we cycle through various stages, including REM (rapid eye movement) and NREM (non-rapid eye movement) sleep, multiple times. This cycling is indicative of an ultradian rhythm, with each complete cycle typically lasting about 90 to 110 minutes in humans. Furthermore, these sleep cycles are essential for processes such as consolidating memories and restoring the body's energy levels.