In the absence of external cues, few biological clocks are accurate-for example circadian rhythms are not set to exactly 24 hours, but can be several hours longer or, more frequently, shorter. Ln normal life, they are kept in synchrony with the environmental cycles by external cues that reset the biological clock each day. the most common circadian cue is photo-period-the most invariant of the the environmental cues. The importance of a good circadian clock to a bat is obvious. With insect abundance peaking at dusk and dawn it’s no good if the bat sleep in and misses them. On the other hand it does not want to arouse too early and waste energy. Some of the best evidence for circadian clocks in bats comes from studies of hibernating bats.