Figure 52.11 Circadian Rhythms Are Entrained by Environmental Cues The activity–rest cycle of a laboratory mouse (a nocturnal animal) responds to the light–dark cycle under which it is kept. The gray bars indicate times when the mouse is running on an activity wheel. Two days of activity are represented on each horizontal line. Thus, day 1 is 0–24 and day 2 is 24–48. Then on the next line day 2 is replotted as 0–24 and day 3 is 24–48, and so on. This double plotting is merely to make the pattern easier to see.