Beavers operate with a quite predictable circadian rhythm. You can pretty much set your clock to a beaver’s schedule. In the past, when I’ve spent endless hours getting to know one of the pairs of beavers in Centennial Woods, I’d show up in the late dusk and wait for them to emerge from their dens. Despite not having watches, the beavers were remarkably consistent, bubbling up to the surface of the pond within 3-5 minutes from the night before.
But in the winter, when the beaver’s dark world becomes enshrouded by the ice, the cover of darkness is far less important that energy conservation. Beavers store their winter food supply in underwater caches, which are securely tucked away under the frozen surface of their pond for easy access in the coldest months. A beaver will leave its dark lodge through an underwater tunnel, swim through the dark water (ice with a layer of snow on top transmits very little to no light), grab a twig or two, bring it back into their lodge, gobble it up, swim back out to discard the scraps, and go back to sleep. And that’s it for several months each year. There is no day or night, and the beaver slips into what’s called a free-running circadian rhythm – where the intrinsic circadian rhythm no longer adheres to a 24-hour period (source). Left in the dark, the beaver shifts to a 26-28 hour circadian rhythm – though some individuals had cycles >29 hours (source), allowing it to sleep longer, rest more, and save energy.
* Interestingly, the near opposite conditions in the far north during the summer when the sun never sets, we see essentially the same free-running circadian rhythms (source).