Sleep is important for animals to stay healthy and recover from exhaustion. This study evaluated the effect of changes in luminous emittance before “lights-out” on sleeping behavior in dogs. Six healthy dogs (aged 15–51 months; 3 female and 3 male) were exposed individually to each of the 3 different luminous emittances: 600 lux as the control condition, 50 lux as the poorly lit condition, and 1600 lux as the brightly lit condition before “lights-out” (from 4 pm to 9 pm) for 2 days in succession, over a total 6-day period. For each exposure, we observed the dogs' behaviors from 4 pm to 7 am the following day. The order of the 3 luminous intensities was random. Eye condition (open or closed), head position (contact or no contact with the floor or side of cage), posture (6 categories), and behavior (8 categories) were recorded every 15 seconds. Comparisons between the conditions on the number of events spent in each posture or behavior were assessed using a repeated-measures analysis of variance, with post hoc comparisons, and a P < 0.05 was used to assess significance. A paired t test was used to compare eye or head positions under each condition. During the period 4 pm-5:30 pm, there was no difference among the 3 conditions in terms of the number of events each posture or each behavior was shown. From 5 am to 7 am, after exposure to poorly lit conditions, the number of events involving lateral recumbency was significantly greater than that in the control (Tukey, P < 0.05). From 5 am to 7 am, after exposure to poorly lit or brightly lit conditions, the number of events involving eyes closed was significantly greater than that spent aroused (t test, P < 0.05), but otherwise there were no significant differences compared with the controls. These results suggest that changes in luminous intensity before night time might influence sleep quality in dogs.