I find it amusing to jump straight to the "because some species lack watches." It isn't like humans started with them. Kids aren't even really able to use a clock for quite a few years.

Article is still neat, mind. I am curious why it is not more compelling to think in terms of reserves and duty cycles. Build up enough energy to get you through periods of needed energy and you will settle on a cycle that matches when energy is available. At least, if you want to minimize complete depletion. Which is about the only thing I would expect evolution to fully avoid. Or, at least, the ones that didn't will have died off.