Nocturnal behavior is a critical life-history trait associated with the evolutionary success of bats. Avian predation, competition with birds, and hyperthermia may explain the origin of this behavior, but it is difficult to assess the validity of these hypotheses due to the rarity of diurnal activity in bats. In Gorgona National Natural Park, Colombia, the population of the lesser sac-winged bat (Emballonuridae: Saccopteryx leptura) has an unusual daylight flight activity, which provides an opportunity to learn about the origin of nocturnality in bats. We described the daylight activity of S. leptura in Gorgona Island and explored the influence of environmental factors at different spatiotemporal scales, potentially involved in this particular behavior. We registered the variation of light intensity and temperature across days, months, and different vegetation canopy covers and assessed their effects on the number of daylight flights. We found that the daytime activity of this insular population increases with vegetation cover, decreases with light intensity and temperature, and varies seasonally. These responses of flying activity to environmental temperature and light intensity seem to be related to avoiding the risk of overheating. Furthermore, bats were also active at night, and daytime flights included echolocation and feeding, which suggest that daytime activity complements but does not replace night-time activity. Consequently, our results provide empirical support for the notion that the nocturnality in bats can reflect the actions of avoiding overheating during the day.