Circadian Rhythms

The circadian timing system is a major determinant of daily variations in subjective alertness, neurobehavioral performance, and sleep. The early evidence for this was derived from longterm sleep deprivation experiments carried out in the 1970s. In those experiments, alertness and performance exhibited rhythmic variations over the course of the sleep deprivation, with a period close to 24 hours, superimposed on a steady deterioration of alertness and performance, attributable to sleep loss. The notion that sleep, alertness, and neurobehavioral performance are determined by the interaction of two processes, a circadian and a sleep-wake dependent process (sometimes referred to as a homeostat), is now widespread.

Studies carried out in the late 1970s found that spontaneous sleep duration depends primarily on the phase of the circadian timing system at bedtime, rather than on the length of prior wakefulness. Those studies involved young subjects free-running in temporal isolation, with the subjects allowed to self-select their bed and wake times. Results from such studies consistently documented that spontaneous sleep duration was longest when bedtime occurred near the circadian phase at which temperature peaks, and spontaneous sleep duration was shortest when bedtime occurred closer to the circadian phase of the temperature nadir (which under normal entrained conditions occurs 2-3 hours before usual wake time). Thus, the spontaneous self-selected duration of sleep episodes begun at the peak of circadian sleep tendency (at or after the temperature nadir) were actually cut short by the rising portion of the wake propensity rhythm (the variation in the likeliness of waking up or of remaining awake), whereas those begun at the nadir of circadian sleep tendency (near the temperature crest) were extended by the rising portion of the sleep propensity rhythm. Ironically, this occurred because sleep propensity was greatest just after the endogenous circadian temperature nadir and minimal near the endogenous circadian temperature peak. In fact, results from forced desynchrony studies carried out on young adults during the 1990s suggest that proper alignment between the timing of sleep and the timing of circadian rhythms is even more important for sleep consolidation than previously thought.