Window of Circadian Low - The human biorhythm goes through different cycles throughout the day. “Sleep pressure”, or the need for sleep, expresses itself at different levels depending on the time of day. There are times when a person can hardly sleep at all and if they do, then the sleep provides a substandard quality of recuperation. On the other hand, there are also times – in particularly between two and six in the morning – in which the urge to sleep is especially strong. And the restorative effects are also much better if one sleeps during this period. This time period at night is referred to as the Window of Circadian Low (WOCL).