Figure 29.18: Control of Activity by a Circadian Biological Clock The rest–activity cycle of flying squirrels (Glaucomys volans) responds to the light–dark cycle under which the squirrels are kept. The squirrels are nocturnal. All the data shown are for a single individual studied (A) with a light-dark cycle and (B) in continuous darkness. Each horizontal line corresponds to one 24-hour day. Where a horizontal line is shaded gray, the squirrel was running. In each set of data, ten successive days, numbered 1 to 10, are portrayed from top to bottom.