Have you ever wondered how animals who live near the poles know when to sleep, what with the long seasons of mostly light or mostly dark days? Me too!
Before I get into that, let me explain how sleep/wake cycles work in the animals that live in normal parts of the earth. Circadian rhythms are biological oscillations that maintain an intrinsic periodicity of about 24 hours in constant (i.e. 24 hours of dark) conditions–to simplify: they start over again each day. Some circadian processes may include:
- behavior (sleep/wake, brain activity, physical activity)
- physiology (hormone levels, metabolism, body temp)
- cellular functions (cell cycle and maintenance)
- gene expression (e.g. transcription)
These rhythms can all be entrained, or re-set, by cues such as light/dark (LD) cycles, temperature fluctuations, and feeding times. One hormone that’s very important for regulating circadian behavior and physiology is melatonin. It is produced rhythmically from the pineal gland, and it helps us get sleepy at night (among other roles). Reciprocally, melatonin secretion is also governed by the endogenous circadian clock; that’s why we humans can pretty well maintain our sleep schedules as day lengths fluctuate throughout the year. Even if the sun was blotted out for a day, our melatonin levels would still rise at the time we normally get ready to sleep.
However, and this is a side note: melatonin levels do decrease with exposure to light even late at night. That is why some researchers have advised against staring at a bright computer or phone screen past your normal bedtime–this suppresses melatonin secretion and may make it harder for you to sleep. The consequences of the resulting sleep deficit may include increased stress, lowered immune system, impaired cognitive function, and other health issues. Of course, do what I say, not what I do–I’m up writing this at 1 a.m.
Back to the point. Some scientists studied reindeer (or caribou, as many of us know them) to help answer the aforementioned question concerning their sleep regulation. Reindeer are native to the far north, including Arctic and subarctic regions, so they definitely have to spend their winters and summers in extreme (or “polar”) light/dark conditions. The scientists found that reindeer melatonin levels do not oscillate during midwinter. Experimentally, melatonin secretion was found to simply rise in the dark and fall during light exposure. Moreover, these reindeer do not have much of an endogenous clock at all during the most extreme times of the year. What does this mean for their behavior during midwinter and midsummer? Well, they definitely don’t sleep for eight straight hours like we do. Basically, their sleep rhythms are regulated by feeding. They eat, then they sleep or nap. And they eat about 8-10 times in one 24 hour period.
Interestingly, reindeer do show circadian rhythms in the spring and autumn, when there is a more normal LD cycle. But they’ve adapted to life in constant light/dark conditions so their clocks are very flexible. You might say that their circadian rhythms take summer and winter vacations.
Anyway, let’s just hope that Santa gives his reindeer some snacks and nap breaks on this long Christmas Eve night. Happy Holidays!