Figure 6. Effect of interaction between circadian phase of the start of the sleep episode and time since the start of the sleep episode on wakefulness within quarters of the scheduled sleep episodes in older (•) and young (^) subjects.

Sleep episodes were sorted according to the circadian phase of the core body temperature rhythm at lights out (resolution, 60 deg). A, sleep episodes starting at 180 deg (range, 150-210 deg); B, 240 deg (range, 210-270 deg); C, 300 deg (range, 270-330 deg); D, 360 deg (range, 270-390 (i.e. 30) deg; E, 60 deg (range, 30-90 deg) F, 120 deg (range, 90-150 deg). Wakefulness is expressed as percentage of recording time. Insets reflect the circadian component of the body temperature rhythm in young and older subjects and boxes indicate (approximately) the part of the circadian cycle traversed from start of sleep to end of sleep.