Fig. 4. An `anticipatory' rise or fall in the level of serotonin in the brains of lobsters is revealed when the means of serotonin (5-HT) content in whole brains (data from Fig. 3A) are pooled and plotted against the hours since light-on or light-off in a three-way ANOVA. Little change in serotonin levels follows light-on or light-off either in the 0–4 or 4–8 h time periods after they have occurred. In the 8–12 h period, however, the levels in the animals approaching dusk increase and those approaching dawn decrease. These changes occur well before the light-on or light-off transition.