What it is:
Up until relative moments ago on the evolutionary timeline, our internal clocks were guided by the availability of natural daylight. At higher latitudes where the difference in daylight hours throughout the seasons is more extreme, this would have pushed residents closer to a state of what we might think of as “hibernation-like” in order to survive. As mammals who lack a thick layer of insulation and rely heavily on our sense of vision, we are inherently more vulnerable during those long, dark winter nights.
Fast-forward to modern day and one might stop and marvel for a moment, not only at the electric lighting that enriches our lives, but also all of the other amenities that most of us take for granted. If we have the resources available, we can harness technology to avoid discomfort and even travel to milder climates to escape the depths of winter. But can we ever trick our internal cellular clocks entirely? Can we ever completely uncouple our biology from nature’s rhythms? Many ancient holistic health practices encourage us to take a symbiotic approach, adapting our routines as the seasons ebb and flow. This includes what we eat and how we might spend our time, and sleep is an integral part of that. If we pay close enough attention, we might detect subtle changes in our sleep patterns this time of year, and maybe that’s not such a bad thing.
The purported claims:
Office-based work doesn’t require seasonal variations in hours at the desk due to weather and natural lighting, but we may still experience internal shifts in energy and wakefulness with the seasons.
Melatonin secretion patterns can vary throughout the year due to the number of daylight hours available.
During long winter days, non-light-based time cues are critical for keeping our circadian rhythms on track.
What the science says:
A defining feature of being human is that we are always changing. We learn and grow with time, hopefully working towards meaningful goals and bettering ourselves, but we also ebb and flow in a non-linear fashion across a longer-term trajectory. We can observe this across the business week, bookended by slower-paced weekends, but we also see this on a larger scale across the calendar year. Geographical location profoundly influences when we can enjoy the outdoors versus need to seek shelter from the elements, but ties with the academic calendar, cultural and national celebrations, and industry-specific rhythms also shape our individual ebb and flow.
In addition to defining a season by its level of busyness, the photoperiod, or number of hours of daylight in a season, can significantly influence our mood, energy levels, behaviors, and sleep quality. Studies that have tracked people across the year have noted higher caloric intake, blood pressure, and cholesterol in the autumn months. Changes in mood and the prevalence of seasonal affective disorder are also well described. We typically think of the “off” season as being colder, darker winter, but for individuals in climates with extreme summer heat, the season in which the affective disorder strikes can be reversed. If it’s too hot to go outside in the summer, exposure to light may, counterintuitively, be reduced during these months instead.
What we know
Research indicates that our sensitivity to light depends on recent light exposure. In other words, a day spent in a dim office increases our sensitivity to light that night. In contrast, a day spent outdoors in plenty of natural light offers some protection against exposure to artificial light that night. Numerous headlines have warned of the deleterious effects of light at night in recent years. Yet, these articles often miss an essential caveat: maximizing the amplitude between how bright our days shine and how dark our nights fall is crucial.
There are, however, individual differences in sensitivity to light. During the day, we feel this as we reach for our sunglasses, and at night we can perceive that sensitivity in how alerted we are by overhead lights and bright screens, in turn disrupting our drive for sleep. Interestingly, some studies have shown that in addition to the variability in sensitivity to light that is experienced between individuals, there may also be changes within individuals according to the time of year. The mechanisms that drive this need further investigation, but this is likely related to who is most vulnerable to seasonal affective disorder and seasonal sleep disturbances.
Several researchers have studied sleep and mood among individuals living in polar conditions, where seasonal extremes produce near-constant darkness or daylight. Extremely long days can negatively impact sleep by partially suppressing the sleep-promoting hormone, melatonin. However, melatonin patterns can also become disrupted in the absence of strong enough light signals during the day. Interventions which focus on administering bright artificial light during the day generally lead to better sleep at night, in absence of proper daytime cues from the sun.
Scientists have also investigated how melatonin patterns change under less extreme seasonal differences by bringing people into the lab to experience either 8 hours of light and 16 hours of darkness, or vice versa, similar to the natural change in photoperiod in Seattle, WA. When there was 16 hours of daylight, sleep duration was shorter and melatonin secretion shifted later at night. Moreover, the decline in melatonin towards morning manifested as a gradual tapering off. In contrast, during the simulated winter condition, melatonin began being synthesized and released earlier at night, but dropped off much more sharply in the second half of the night. The ways that this might translate to real-world sleep are as follows:
In the Summer, we might not feel tired until later at night. In particular, we see this during daylight savings time.
In the winter, the sharp drop in melatonin in the second half of the night might have us waking up from sleep in the very early morning (earlier than we typically want to) and feeling quite alert. A person might find that they almost want to sleep in two shifts (called “biphasic sleep”), compensating for a middle-of-the-night awakening by getting up a bit later the next day.
Drastic changes in sleep across the year seem to be strongly linked with latitude. There has been much debate about how humans slept in pre-industrial societies, and the length of the photoperiod during seasonal extremes likely mattered significantly. Today, we can gain some insight into how our ancestors might have slept by studying nomadic tribal populations such as the Hadza in Tanzania or Tsimane of Bolivia. In these populations, sleep does vary throughout the year, but there isn’t evidence of a biphasic sleep pattern, likely because these individuals live much nearer to the equator. In these regions, indigenous tribal peoples generally fall asleep about 3 to 4 hours after sunset and wake before the sunrise. Afternoon naps during the warmest part of the summer day (which also coincide with the natural circadian dip in energy) are also fairly common.
In a study of 500 college students in Seattle, WA, individuals shifted their bedtime and wake time about 30 minutes later during the winter, as compared to the summer. In another study that evaluated nearly 300 in-lab polysomnography records that had been collected over the course of a year, it was found that winter sleep was about an hour longer, rapid eye movement (REM) sleep appeared about 30 minutes earlier in the night, and was 30 minutes longer, while the proportion of deep sleep across the night remained consistent. It’s unclear why there is this variation in REM sleep, but given that it is important for certain kinds of memory and emotion processing, as well as dreaming, some people might find that winter yields greater introspection and creativity. Sleep tracker data might show these artifacts despite modern lifestyles and the use of electric lighting.
A group of researchers in Germany are currently tracking a group of people across an entire year, and will collect data on various metabolic markers and measures of sleep and mood. There is much more still to learn about the ways we likely continue to be intrinsically linked with nature’s rhythms, no matter how urban our lifestyles are.
Our take:
Research suggests that we are inextricably linked to seasonal changes in daylight hours that may result in an ebb and flow in our sleep-wake patterns throughout the year. Where light/dark signals are trickier to synchronize with our need for strong wake/sleep-promoting cues, we can turn to other important time cues, including meal and exercise timing to keep us on track. The brain is a pattern predictor, so it helps to give it behavioral patterns to predict. This also includes work and social hours, as well as other activities.
The stronger our exposure to bright morning light, the earlier we will fall asleep that night. Begin your day by opening all the curtains and blinds upon waking, and ideally take a short walk outside before your workday begins. This checks two boxes by synchronizing your circadian rhythms and warming up your muscles.
Throughout your day, work near a window or in a room with smart lighting and take breaks outdoors whenever possible. If you live at a higher latitude, use a light therapy box to compensate for the lack of morning sunlight during the winter months. Focusing on these strong morning light cues is a more proactive and empowering approach to supporting your sleep than fearing exposure to any artificial light at night, and arguably more effective. That being said, it certainly doesn’t hurt to keep the lighting soft in the hours preceding sleep to bolster circadian health.
Will this benefit me?
Embrace seasonal nuances in sleep-wake timing and patterns if you can do so without any negative consequences and find that it supports your overall wellbeing. Akin to seasonal eating and movement, if you can take more time to rest during the winter, be sure to do so. However, be mindful of any influences that distance you from your health goals, such as overindulging in unhealthy foods or alcohol.
Still curious to try it? If you do, here’s what to keep in mind:
It can be hard to know how to get it right when you’re trying to balance adequate light exposure for supporting circadian rhythms, sleep, and vitamin D synthesis while minimizing the risk of skin cancer and eye damage. We each vary in our sensitivity to ambient light, but try your best to strike a balance and speak to a trusted healthcare provider about any specific concerns.
References and additional materials:
Light exposure in polar regions meta-analysis - https://pubmed.ncbi.nlm.nih.gov/38253963/
Chronotype delayed in polar dark - https://pubmed.ncbi.nlm.nih.gov/37743400/
Protocol for year-long tracking study - https://pubmed.ncbi.nlm.nih.gov/39039613/
Seasonal variations in how our melatonin system responds to light - https://pubmed.ncbi.nlm.nih.gov/37987395/
Seasonal variations in sleep pre-industrial society - https://pmc.ncbi.nlm.nih.gov/articles/PMC4720388/
How melatonin secretion varies across the year - https://pubmed.ncbi.nlm.nih.gov/37694335/
Against sunscreen absolutions - https://www.theatlantic.com/magazine/archive/2024/06/sun-exposure-health-benefits/678205/
Sleep timing of college students in Seattle - https://pubmed.ncbi.nlm.nih.gov/36404490/
Seasonality of human sleep - https://www.frontiersin.org/journals/neuroscience/articles/10.3389/fnins.2023.1105233/full
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