Abstract
© Cambridge University Press 2013.It would be extremely unusual, not to mention highly inconvenient, if everyone woke up and went about their daily routines at the same time. Fortunately this is not the case, and humans display a wide range of sleep–wake timing preferences. Some of us like to wake up and get things done in the morning (so-called larks, or morning types), others prefer to be active later in the day and night (owls, or evening types), and many are in between or a mixture of the two. The range in sleep–wake timing is considerable and differences in preferred bedtime and wake time can be as much as 2–3 The on average between morning and evening types [1], and in circadian rhythm sleep phase disorders, bedtimes can range from 7–9 p.m. (advanced) to 2–6 a.m. (delayed) [2]. It has often been assumed that diurnal preference (morningness versus eveningness) is not an acquired characteristic but relates to biological factors involved in the circadian timing system that regulates the optimum times for waking performance and sleep–wake timing. However, current understanding of factors influencing variation in sleep–wake timing and optimal timing of waking performance emphasizes the interactive contribution of social factors, such as work schedules and leisure time, and biological factors. Underlying biological factors include the timing (phase of entrainment) of the endogenous circadian rhythmicity relative to clock time, and the light–dark cycle [3]. The phase of entrainment is determined by the intrinsic period of the circadian clock, as well as sensitivity to the effects of light on the circadian clock. In addition, sleep homeostatic mechanisms also play an important role in sleep–wake timing. This implies that diurnal preference could be related to any of these three main factors: circadian period, light sensitivity, and sleep homeostasis.