Other biological cyclesThe sleep cycle is itself embedded in a larger biological cycle called the circadian rhythm. The circadian rhythm controls our sleeping and waking, as well as regular cycles of drowsiness and alertness throughout our day.In turn, the body's biological clock and the physiological activity it controls are influenced by the seasons, especially the length of the daylight hours. Many people experience a depressed mood and lower energy levels during the gloomy winter months, and may feel a need for more sleep than during the brighter summer months. So it may be impossible for us to understand sleep fully until we understand the purpose of these larger cycles. |