Figure 1.

The mean times for the morning and evening type groups of their typical sleep period (horizontal shaded bars), of their subjective sleepiness minimum (SSmn) and maximum (SSmx), objective sleep propensity maximum (the maximum sleep in the 15-minute hourly sleep opportunities, SPmx and sleep propensity minimum (SPmin), dim light urinary melatonin onset (DLMO), and core temperature minimum phase, Tmin (down arows). The same phase markers for the two groups are connected with thin dashed lines so that the slopes of the connecting lines can indicate the relative differences between the two groups. It can be seen that the circadian and sleep timings are all about 2–3 hours later for the evening types. However, the subjective sleepiness phases show the greatest differences between groups. The morning types feel least sleepy in the early afternoon and the evening types least sleepy or most alert in the late evening, only a few hours before their typical bedtimes. The morning types then feel most sleepy before their Tmin (down arrow) but the evening types feel most sleepy after their Tmin at about their typical wake time.