Figure 4.

The effect of the sleep/wake cycle (solid line) and the endogenous circadian system (dotted line) on circulating levels of melatonin, cortisol, growth hormone (GH), prolactin and thyroid stimulating hormone (TSH). The solid line represents measurements taken whilst subjects maintained their habitual sleep/wake cycle including daytime activity and meals and nighttime rest and fasting. The dotted line represents data collected while subjects were under constant routine conditions including constant rest, semi-recumbent posture, wakefulness and hourly isocaloric snacks. For the sleep/wake cycle only, the vertical lines of the rectangle represent relative clock hour of habitual bed time (left line) and habitual wake time (right line) and thus the rectangle indicates scheduled nocturnal sleep. Note that the rectangle is not applicable to the constant routine conditions in which the subjects remained awake. The melatonin data were collected under dim light conditions (<3 lux during wakefulness and <1 lux during sleep periods) and are based on data from Gooley et al. (2011). Cortisol, GH and TSH data were collected under dim (<1 lux during sleep periods) and room light (~150 lux during wake periods) conditions and are based on data from Czeisler and Klerman (1999). The prolactin figure was based on data courteously provided by C.A. Czeisler. Copyright held by the Division of Sleep Medicine, Department of Medicine, Brigham and Women’s Hospital, and Harvard Medical School, Boston, MA. Reprinted with permission.