Figure 4

Phase shifts of acrophase of aMT6s rhythm. A. Phase shifts in the aMT6s circadian rhythm resulting from light stimuli are shown for 106 participants. The ordinate shows the shift in h of the aMT6s acrophase, computed as the baseline aMT6s acrophase minus the acrophase after the 3 bright light treatments. Thus negative shifts indicate that the follow-up acrophase was later than the baseline acrophase, i.e., delayed in clock time. The abscissa represents the timing of the midpoints of the 3-h light stimuli, as referenced to the baseline aMT6s acrophase. Stimuli given with an abscissa near 0 were approximately centered at the baseline aMT6s acrophase. Black circles represent phase shifts of individual young adult participants, and red triangles represent phase shifts of older participants. The solid black horizontal line shows the mean of all points, approximating the phase shift resulting from the circadian free-running component. The black dashed and red lines represent the trends from 5-point moving averages for the young and older groups. Rectangles illustrate the average actigraphic home sleep times for the young and older groups, referenced to their aMT6s acrophases. B. The phase shifts in aMT6s acrophase were averaged to show the mean ± 1 SEM for 2-h bins of time-of-stimulation referenced to the aMT6s acrophase. "The abscissa (Circadian Clock Time) references the midpoint of 3 h light stimuli to the time of the baseline aMT6s acrophase, and then displays the environmental time scale corresponding to when the mean aMT6s acrophase occurred at baseline. Thus the Circadian Clock Time abscissa (Figs 4-6) also represents our best estimate of the mean environmental clock time at which bright light stimuli occurred, adjusted for each participant's baseline circadian phase (aMT6s acrophase or onset). The asterisks illustrate the mean aMT6s acrophase times for the young and older groups.