Phase resetting responses to a 3-cycle 5-hour stimulus in young adults. The magnitude of the phase shift (in hours) is plotted with respect to the relative duration of the bright light exposure. The 0% group was exposed to complete darkness throughout the 5-hour stimulus and the 100% group was exposed to continuous bright light throughout the stimulus. Two groups received intermittent bright light exposure during the 5-hour stimulus: 5.3 minutes of bright light interspersed with 19.7 minutes of darkness (31% relative duration group); or 46 minutes of bright light interspersed with 44 minutes of darkness (63% relative duration group). Each point represents the response from 1 subject, and the horizontal bars represent the median phase shift for the group. Figure reproduced with permission from Reference 70 [Rimmer DW, Boivin DB, Shanahan TL, Kronauer RE, Duffy JF, and Czeisler CA. Dynamic resetting of the human circadian pacemaker by intermittent bright light. Am J Physiol 2000; 279(5):R1574–R1579]