Many plants and animals not only engage in a cycle of daily activities (opening of flowers, waking, feeding, etc.) but also in seasonal activities.The most reliable clue to the change of season is length of day (temperature is far less reliable!). The farther a plant or animal lives north or south of the equator, the more pronounced the changing ratio of daytime hours to nighttime hours with the changing seasons.

It is not surprising then that both plants and animals mainly depend on photoperiod to prepare for changes in seasonal activities.

And what better way to measure the relative length of day and night than by enlisting the machinery by which circadian rhythms are entrained?

One promising model for photoperiodic responses in plants depends on circadian rhythms. Link to a discussion.

As for animals, recent work with Drosophila suggests that this animal uses two circadian clocks to monitor the changing length of day and night.
  • an "evening" clock that takes over in the long days of summer.
  • a "morning" clock that is inhibited by light but takes over when the nights are getting longer;
The molecular machinery (Cry, Tim, Per, etc.) for each clock is confined to separate neurons in two different parts of the brain.

In these experiments, Drosophila is using the two clocks to adapt daily — not seasonal — cycles of activity to the changing seasons. But this machinery for measuring photoperiod could enable them to prepare for seasonal changes in activity, e.g., to stop forming eggs at the end of the summer. (However, other studies examining such seasonal changes in Drosophila find that the photoperiodic response is independent of circadian responses. So we must await more experiments to resolve the question.)