The change in heartbeat rate of Tg((gal4)cmlc2:GFP) larvae from 3 dpf to 5 dpf in different lighting conditions. Larvae were maintained in: (A) alternating light and dark (LD) conditions continually; (B) LD until 85 hpf, after which they were placed in constant dark (DD); or (C) constant light (LL) starting as soon as the eggs were fertilized. In each case, the heartbeat rates were recorded every 6 h from 3 dpf until the end of day 4. In these graphs, the yellow segments indicate the light phases; the dark grey segments indicate the dark phases, and the light grey segments indicate the phases when the entrained (now free running) zebrafish circadian clock would have experienced light. In (A–C), the peak and trough