Ultradian Rhythms

Within humans, daily cycles of wakefulness and sleep, follow a basic 24 hour cycle or circadian rhythm. However within the sleep portion of this cycle another type of rhythm, an ultradian rhythm exists. An ultradian rhythm is a biological rhythm that occurs with a frequency of less than 24 hours. Sleep is composed of several repetitive cycles of about 90 minutes in length. The sleep cycle is composed on two types of sleep: REM (Rapid Eye Movement) and non-REM sleep. In REM sleep the individual undergoes a relatively brief period of vivid, erratic dreams. When the brain is monitored by EEG (electroencephalogram) the brain activity is similar to the EEG of a person in a state of wakefulness, however it is more difficult to awaken sleepers in REM sleep, indicating that REM sleep is still a type of deep sleep. [23]. Physiologically the body picks up the pace as well; there is an increase in heart rate , eye movement, and respiration. In non-REM sleep, there are four stages progressing into a deep sleep or slow wave sleep. In this slow wave sleep the body physiologically slows down and the EEG takes on a new amplitude and appearance. So within the 90 minute sleep cycle the body undergoes at first a cycle of non-REM sleep, followed by a shorter period of REM sleep. These cycles then continue throughout the night with the REM period getting slightly longer as the evening progresses.

Earlier in the document (The Human Clock Chapter) there was a brief discussion on how hormones and the endocrine system exhibited a circadian rhythm. If one takes a closer look at many of those hormones it becomes apparent that they also exhibit an ultradian rhythm. Most hormones are secreted into the bloodstream in a pulsatile manner rather than in a continuous fashion. This is not to say that a hormone can not have a peak or average plasma concentration that persists at one point in the day more than any other moment. Just as you have 90-minute sleep cycles with the period of sleep/wakefulness, so does a hormone have a group of individual pulses making a smaller part of the overall circadian cycle. Growth hormone (GH) in humans would be a good example of this. Growth hormone has a pulse of hormone every three hours, yet overall its period of highest concentration is found at night[29].