Figure 2.

Mean phase angle of entrainment (A, B) and its within-subject variance (C, D) are depicted for the activity onsets associated with each of the two bifurcated activity bouts (blue and red lines) under lengthening (A, C; n=8) and shortening (B, D; n=10) LDLD T cycles. For phase angle of entrainment, activity onset is plotted in hours relative to dark onset (ψ_A,D) such that activity onsets following dark onset are indicated by positive values. The between-subjects variability in this measure (one standard error of the mean) is represented by correspondingly colored horizontal lines to the far right in panels A and B, displayed only at hourly values of T. The variance in phase angle represented in C and D was calculated over 5-7 days for each bout in each animal and then averaged across subjects. According to classical entrainment theory, entrainment results when the daily phase shift (Δϕ) equals the difference between the free-running period and zeitgeber period (τ - T). Consequently, progressively larger phase delays and advances are required when T lengthens and shortens, respectively. Based on the extensively characterized light pulse PRC in hamsters, increased phase delays under lengthening Ts should be achieved by light falling later into subjective night: i.e. if the L/D transition is equivalent in effect to the light pulse used to derive a PRC, activity onsets should occur progressively earlier relative to dark onset. Despite successful entrainment, this predicted relationship is not observed. Similarly, under shortening Ts, increased phase advances should be achieved by light falling progressively earlier into late subjective night, and activity onsets should increasingly lag dark onset, so that phase advances will increase as the timing of the D/L transition moves earlier in the subjective night. Such a relationship is apparent only up until ~ T = 22 h.