Figure 4. Difference in CIL, not absolute CIL level, is important to drive circadian rhythmicity in CH/CL cycles.

(A) Flies were first exposed to constant light (LL) for 2 days and then entrained to CH/CL cycles for 7 days, followed by constant CL conditions for 7 days. Daily activity profiles of CS flies from day 6 of CH/CL to day 3 of CL/CL are shown. Activity peaks became lower and disappeared beginning on day 3 of constant CL. The numbers of CS flies used for the analyses were 56. (B) Averaged actograms of flies of indicated genotypes are shown. Each row of the actogram was double-plotted. The numbers of flies used for the analyses were 56 (CS), 63 (w¹¹¹⁸), and 18 (per⁰¹). VIL and CIL of 10,000 K (CH) light are increased to 1,570 lx and 1,734 blx, respectively. Those of 2,000 K (CL) light are increased to 1,470 lx and 338 blx, respectively. Dark blue and red horizontal bars indicate brighter CH and CL, respectively. Flies were first exposed to constant light for 2 days and then exposed to 12 h:12 h cycles of brighter CH/CL. Whereas CS and w¹¹¹⁸ flies show circadian rhythmicity, per⁰¹ flies show no rhythmic pattern, which is similar to observations made previously for CH/CL cycles with low light intensity. (C and D) Entrainment indices were calculated for flies of each genotype under brighter CH/CL cycles using time windows ZT6–12 (C) and ZT9–15 (D). ZT, zeitgeber time. Values represent mean ± SEM and are compared by pairwise t-test. **p < 0.001. *p < 0.05.