FIGURE 13-3 Free-Running Rhythm in a Human The record for days 1 through 3 shows the daily sleep period under typical day–night conditions. The record for days 4 through 20 shows the free-running rhythm that developed while this participant was isolated in a bunker and allowed to control day and night lengths. The daily activity period shifts from 24 hours to 25.9 hours. On days 21 through 25 the period returns to 24 hours as the participant is again exposed to a natural light–dark cycle.

Data from J. A. Hobson (1989). Sleep (p. 33). New York: Scientific American Library.