Estimation of rhythm peaks for phase response curves. (a) Averaged locomotor activity (mean ± SEM, the latter indicated by dots), plotted for 6 males that had been individually monitored for locomotion. The photic conditions, indicated by the shading as in Figure 1, involved ca. 6 days of 12-h:12-h LD cycles followed by about the same number of DD ones. A 10-minute light pulse, timed according to the asterisk placed within a thin white stripe, was administered 4 hours after the final lights-off, i.e., early in the subjective nighttime. (b) Another set of 6 males maintained in the same conditions, except that they received no light pulse after proceeding into DD. (c) The data sets from (a) and (b), re-plotted together after smoothing them by application of a low-pass filter set with a cutoff of 12 hours. This treatment facilitates identification of peaks in both data sets, as indicated by the asterisks for the pulsed group and the crosses for the no-pulse group. (d) Difference between the two groups, plotted as changes in hours vs. time for each peak over the course of the LD -> DD monitorings. The timing of a given peak for the no-pulse group was subtracted from that for the light-pulsed group, resulting in depiction of a net phase delay for the latter flies in DD. The 3 rightmost points on the abscissa indicate stabilization of 3.5-hour, light-induced phase delay.