Free-Running Circadian Rhythms

If you took off your watch and stepped into a box that buffered you from any environmental cues that would hint at the time of day – it’s not just light/dark cycles that due this, traffic noise can do the same – you’d probably go mad (source). If you could somehow stave off the psychological impacts of sensory deprivation, you’d soon fall out of touch with your watch by – wait for it – 1 hour (source)!! That’s right, the internal human clock is set on a roughly 25 hour cycle (not surprisingly, circadian rhythms tend to be slightly longer than a solar day). So there’s that extra 1 hour of sleep I’ve been craving. I’ll just have to move my whole family into a sensory deprivation tank….

Circadian rhythms essentially anticipate consistent, external conditions synced up the earth’s rotation. These can also help cue organisms into seasonal changes for timing migrations, hibernation, breeding, etc. Pollinators like honey bees can forage when nectar sugar concentration is the highest. CAM plants like pineapple can finish the photosynthetic process at night when temperatures and risk of water loss are lowest. Virtually all organisms from plants to cyanobacteria, even your gut flora/fauna, (source) experience a world with punctuation marks every 24 hours (though sadly, archaebacteria lack this adaptation).

Circadian rhythms even persist in environments that remain relatively stable through time. Cave-dwelling species often exhibit extreme modifications to suit their extreme environment. After thousands of generations in caves, the cave-dwelling millipede, Glyphiulus cavernicolus, has lost most pigmentation and has degenerate eyes. However, when a researchers brought a bunch of these millipedes up to a lab and exposed them to light/dark cycles, over half of the millipedes began to show circadian rhythms (source). The mussels found in deep sea thermal vents have circadian rhythms, which may be linked to tidal shifts or could be a vestigial behavior (source).

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).