The winter solstice rolls us into the second half of the solar year. Trees surrender their fiery leaves to frozen dewdrops on bare branches. Shorter periods of daylight and longer portions of night lure thick socks and hot water bottles out from a deep drawer of winter clothes. Hearty stews simmer and fires frequently flicker as the season shifts. Animals seek warmth and reprieve from the cold by “… migration, hibernation and resistance (tolerance)”. For many humans, winter may burgeon these types of adaptations too, encouraging changes to our appetite, our exercise patterns, even our sleep and wakeful behaviours! Unpacking the physiology of this can help us be kinder to our bodies and minds for embracing our inner polar bears during winter months. So, allow me another moment of sleep science nerdery…

An intrinsic, 24-hour, regulatory system is present in each of our bodies. The system is called the circadian rhythm; internal oscillations that cycle around a biological clock. Some molecular mechanisms of the biological clock are retained across plant and animal kingdoms – from the most primitive single-cell organisms like cyanobacteria to human beings. In mammals, the circadian rhythmicity is observed in fluctuations of hormones, sleep-wake cycles and even periods of optimal cognitive functioning. Genetic variables of circadian rhythms are by no means conserved across species, and especially not within the same species.

Your 24-hour structure may differ from your housemate, sibling, or colleague by up to eight hours. This depends on microenvironments within and macroenvironments around the body. For example, your sibling, a shift worker who drives shipment trucks across the country, might wake up at two o’clock in the morning to arrive at their destination by four o’clock that afternoon. Their bedtime may be 6pm. At this time, a night nurse who clocked in at Rahima Moosa moments earlier may be enjoying his equivalent of a “morning coffee” and prepares to work through the night. How can this vary so vastly (and manageably) from person to person?

In brief, the answer is light and chemistry!

The circadian rhythm’s master-regulatory site is called the suprachiasmatic nucleus – a cluster of thousands of neuronal cells in the brain, situated on top of the optic chiasm. The suprachiasmatic nucleus (SCN) is receptive to photic stimulation (think: packages of light called photons sent via the visual system) and chemical (or, neurotransmitter) stimulation. The SCN’s sensitivity to light means that timely exposure to or limitation of daylight can shift the phase of our circadian rhythms; we can feel a desire to sleep earlier and wake up later, like with shift workers.

This is happening across the world as seasons change: a phase shift of circadian rhythm aligning with Earth’s orbital cycles and environmental cues. During winter, the sun sets about 90 minutes earlier than it would in warmer months – it may be a challenge to stay awake until your 11pm summer-bedtime. Similarly, the 5am Club may lose a few committed members (not mentioning any names erhuurhm @me) between May and August. Frankly, if the Sun can go to sleep earlier, so can I! This is an expected adaptation; a sign of your body adjusting well to its circumstance. No big deal!

It is also common for emotional affect to change in winter. Ever heard of Seasonal Affective Disorder? It’s basically the SADs that often come with less sunlight exposure. Our mental health is a huge determinant of our sleep-wake behaviours, too. People who live with a psychiatric illness (or even symptoms of one) are considerably more likely to experience coincident insomnia – up to 90% coincidence! This relationship, though, is no coincidence. Several molecular messengers involved in sleep, wakefulness, appetite control, and mood regulation are in the same class of neurotransmitters called monoaminergic neurotransmitters.

Just as animals do, humans may migrate smaller distances in winter: like, no further than to the snack stash and back. So, maybe we gather a little more belly fat. We don’t have that spring (haha, get it?) in our step. We want to sleep in… Guess what? Colder times of the year welcome a series of natural and protective evolutionary traits to be expressed and experienced. Sometimes that feels like seven extra centimetres between your skin and your belt buckle or admitting you simply must rest for 12 hours every day. Just as we do not look up at leafless trees nor look down at frost on the grass with judgement, there should be no shame around the physical, psychological, or physiological changes that come from seasons changing. If you’re sleeping more and moving less, give thanks to the suprachiasmatic nucleus!