Figure 4. Phase shifts of the master, internal, circadian clock (assessed by the DLMO) due to the 9-hour delay of zeitgebers.

Each dot represents the phase shift of an individual subject from before to after the 3 delayed sleep/dark episodes (from the third to the fourth phase assessment in Fig. 1). The horizontal lines represent the means. The phase delay (mean ± SD) was 2.4 ± 1.3 h for the 23 African-Americans and 3.6 ± 1.0 h for the 22 European-Americans (p < 0.001).