Figure 1 : Protocol diagram.

From: Circadian rhythms of European and African-Americans after a large delay of sleep as in jet lag and night work

Figure 1

The dim light melatonin onset (DLMO) was determined from 30 min saliva samples obtained during the circadian phase assessments (days 1–2, 5–6, 10–11 and 14–15), and was used as a marker for the phase of the master circadian clock. Subjects were kept in temporal isolation and put on a 5-h ultradian light/dark (LD) cycle (LD 3:2) in between the first and second phase assessments. They were put to bed and permitted to sleep during the 2-h dark episodes and kept awake during the 3-h light episodes in relatively dim light (~10–30 lux). The circadian clock cannot entrain to the ultradian LD cycle so it free-runs; called forced desynchrony. The phase shift of the DLMO from the first to the second phase assessment (baseline to final) was used to calculate the endogenous free-running circadian period (τ). After the second phase assessment, subjects were given access to clocks, but we controlled their LD cycle (LD 16:8) and their time in bed, in the dark. On days 7–10, they were put on an 8-h baseline sleep (dark) schedule similar to their sleep schedule at home before entering the lab. A 01:00 to 9:00 sleep schedule is shown. After the third phase assessment on days 10–11 (baseline assessment), the sleep/wake schedule, LD cycle and meal schedule were delayed (made later) by 9 h for 3 days (days 11–13). The clocks in the subjects’ bedrooms were also delayed 9 h, they were changed to Japan time; Japan is 9 time zones west of Chicago. The time line on the top shows Chicago time, and the time line on the bottom shows the corresponding time in Japan. The phase shift of the DLMO from the third to the fourth phase assessment (baseline to final) was used to determine the phase shift of the circadian clock due to the 9-h delay of zeitgebers (time cues).