Does the sleep onset hour affect the length of sleep?
It may but it does not have to. The most powerful factor affecting the length of sleep is the circadian phase. In a normal patient, the circadian rhythm is aligned to the 24 hour period. Thus the sleep onset hour will be directly related to the length of sleep. Maximum length of sleep will occur with the sleep onset falling roughly between 22 and 3 am (depending on the individual rhythm). However, in a free running DSPS patient, this relationship is lost. In DSPS rhythm, the graph of the function of the sleep length vs. the sleep onset hour is horizontal, i.e. the length of sleep does not change (on condition the rhythm is running free without a disturbance).
