SENSING TIME. ‘The other major rhythm is based on the seasonal calendar known as photoperiodism. In this case, the organism responds either to annual changes in day length, or directly to the length of day/night, and uses this information to determine the timing of key events such as migration or the entry into a dormant state. Both circadian and photoperiodic events rely on an ability to precisely measure time.”
http://icb.oxfordjournals.org/content/53/1/131.full