Figure 2

Increasing levels of nocturnal light significantly change mammalian locomotor behavior. Double plotted actograms of two male mice (A) clearly demonstrate that increasing levels of nocturnal light change overall activity patterns in these animals. Time on the x-axis represents local time and the light/dark bar at the top of each actogram corresponds to the light (white) and dark (black) phases of the light cycle. Levels of light during the dark phase (black bars) were systematically increased every 4 weeks as depicted on the right of panel A. No significant differences were found between male and female animals (n = 8 and 7, respectively). Quantification of these patterns demonstrate that while the period (Tau; B) of locomotor activity is unchanged, the total counts (C) and the length of the active phase (Alpha hour; D) significantly decrease and onset error (E) significantly increases with increasing levels of nocturnal light. When animals were returned to < 0.01 lux at the end of the experiment all values trended back toward values seen in the initial < 0.01 lux condition. All values in panels B–D are represented as mean ± SEM. Different letters above each bar represent significant differences with p < 0.05.