Animals are typically active during the day (diurnal) or night (nocturnal), and due to these adaptations, they are more efficient during their selected time of activity. For example, we have poor night vision, so we function best with the adequate light during the daytime. Nocturnal mice sneak around best in the dark, avoiding predators. It makes logical sense to hole up somewhere safe when it is not your preferred time of day, and sleep helps with that conformity.