This happens as the Earth is tilted on its axis at 23.5 degree(1)s and takes 365 days to work its way around the sun. This means that in the northern hemisphere, we are tilted towards the sun in summer and away from it in winter. In this way, less of the northern hemisphere is in the “shade” (the night) in summer at any one time, and so as the earth spins on its own axis, the nights are shorter.
The day with the fewest daylight hours and longest night is December 21st, not long before Father Christmas is conveniently sent to cheer us all up. However, confusingly, the latest sunrise and earliest sunset do not happen on this day, they happen around a week before and 2 weeks after, respectively(2). These uneven changes in the sunrise and sunset(3) are due to the fact that the earth’s orbit of the sun is elliptical rather than circular and because the 24 hour-day invented by us humans does not directly match with the time it takes for the earth to complete its daily spin.
We all like to complain of the days getting shorter and the winter ominously encroaching, as we love to eek out every bit of sun from the late summer evenings. However, many of us also love to complain (we are British after all) about the light mornings awaking us prematurely and the late evenings keeping us up past bedtime in summer. So, it is logical to assume then, that at least in the bleak and cold mid-winter, when the days are short and the light is grey at best, that we can get some good wholesome quality shut-eye; the thick blanket of a winter’s night embracing us with an efficacy that an eye-mask could only ever dream of achieving.
Indeed, it’s true that sleeping in the pitch dark is a great way to obtain good quality and unfragmented sleep. Sleeping with light in the background can lead to awakening when transitioning between sleep cycles, and can even cause other unwanted effects like eye strain(4). However, other factors associated with the changes in light that occur in winter mean that while we may all feel more sleepy in the winter months, we’re not necessarily achieving good quality sleep.
The main reasons why shorter daylight hours can have a negative effect on our sleep are related to our internal biological clocks (also known as our Circadian rhythms) and social jetlag, which is a term used to describe a lack of matching between the daylight hours, our body clocks, and the timings that society expects us to keep.
Melatonin, which is also known as the “sleepy hormone,” is a key player that helps to train our sleep-wake cycles (Circadian rhythms) to changes in our environment like light exposure. Melatonin is released as it gets darker towards the end of the day to prepare the body for sleep. Meanwhile, high light levels during the day suppress the release of melatonin. The low intensity of natural light in winter and the earlier sunsets mean that melatonin is not suppressed and so the melatonin levels in the body are much higher than in the summer(5). This explains why many people claim to be much sleepier during the day in the winter months. Yet, when it actually comes to bedtime and it has already been dark for several hours, our bodies struggle to fall asleep as the distinction between daytime and night-time has been blurred.
In line with this melatonin phenomenon, researchers(6) have shown that workers who experience more hours of sunlight during the morning hours, between 8 am and noon, are more likely to fall asleep quicker and have fewer sleep disturbances. These well-illuminated workers are also less likely to be stressed or depressed. In this way, the dark winter mornings could be increasing our stress levels, lowering our mood and reducing our sleep quality.
Besides the battle that occurs between our own bodies’ hormones and the daylight hours, an additional complication is the demands of the modern working society. Chronobiology is the study of the timing processes of our bodies and how we regulate sleep and wakefulness. Research in this field supports the idea that our sleep preferences change with the seasons and that in the winter months in particular, traditional working routines do not fit well with our body clocks. For example, if our bodies want us to sleep until 9 am because it is still dark outside, but we force ourselves up at 7 am for work, then we have potentially missed out on a whole sleep phase. This means that we don’t obtain enough quality sleep and end up feeling tired during the day.
To examine this idea further, scientists(7) studied different communities that did not have alarm clocks, smartphones or nine-to-five working hours. The researchers found that these people slept up to one hour longer in the winter than in the summer. Further, because these communities were living near the equator, it is likely that removing modern day demands at greater latitudes, such as in the UK, would reveal an even greater difference between “natural” amounts of sleep in the summer and winter.
Other researchers(8) have found links between absenteeism (people being absent from work due to illness) and daylight hours, with more absences occurring in the shorter days of the winter months. Furthermore, January(9) and February are known to be the least productive months for workplaces. It is for these reasons that some groups of experts argue that employers should consider reducing normal working hours in winter to allow their workers to get better quality sleep, which will in turn improve people’s health and wellbeing, reducing sickness and absences, and promoting working efficiency.
After all, differences in sleep quality and duration between winter and summer is not all down to light. There are many other wintery factors that mean we sleep better in the summer. The cold temperatures of winter often mean that we heat our homes and layer our blankets to excess, creating a climate that is too warm to be conducive to a nourishing sleep. Our diets play a huge role too; fattier, heavier and more sugary foods(10), that we often crave and indulge in in winter, have been shown to disrupt sleep. Our reduced desire to exercise and spend time outdoors in the dark, cold and wet is another reason we struggle to sleep at the end of a winter’s day. Finally, stresses and illnesses that plague the holiday season are another factor that our sleep has to contend with in the winter months.
Despite all these obstacles that winter throws at us with the aim of nurturing our ever-growing eye bags, it’s not all doom and gloom. There are many simple and painless lifestyle changes we can make to get that all important beauty sleep. We can make sure we get outside and maximise our natural light exposure, enrich our diet with vitamin D vegetables, and go easy on the fats, sugars and alcohol. We can also ensure our bedrooms are cool, dark and quiet and that we get some exercise during the day, even if its just a short HIIT session or walk.
If these ideas altogether don’t sound like too much fun, and in fact you’d rather cut your losses, feast on the Christmas cake, drink too much, and do no more exercise than walk to the fridge rather than get a good winter night’s sleep, then for better or worse, your only other option is to relocate to the other hemisphere—well—at least for half the year.
References
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