Supporting Your Winter Sleep & Circadian Rhythm - The Apeiron Life Perspective

8 min read

What it is:

The continental United States sits well above the equator, between 25° north at Florida’s tip, to 49° north at the Canadian border. Even Hawaii, with its tropical climate and exotic wildlife, is still 19° north of the invisible line where seasons all but cease to exist and day length, for all practical purposes, remains the same 365 days per year.

The further north (or south, for those in the southern hemisphere) that one travels from the equator, the greater the seasonal extremes in day length. New York City receives less than nine hours of daylight on the winter solstice, compared to just shy of 11 hours in Los Angeles on the same day of the year. At the more extreme end of the spectrum, parts of Alaska receive less than five hours of daylight during winter, which can cause profound changes in energy and mood for those who live there.

Regardless of where you live in the temperate zone, you are likely to be affected by changes in seasonal light exposure. But before you succumb to the winter slump, here are some tools to help you transition and remain energized throughout the darkest months. It starts with putting your shoes on and getting outside.

The purported claims:

Exposure to daylight is essential for brain and body health. It:

Regulates our circadian rhythm
Prevents depression
Boosts alertness
Improves sleep quality
Prevents myopia (nearsightedness)
Allows the skin to synthesize vitamin D

What the science says:

Humans evolved in an environment where daytime spent outdoors was essential for survival, and nighttime was characterized by true darkness and starry night skies. These stark differences in ambient light levels sent important cues to our ancestors' brains about the time of day and the physiological processes most appropriate at a given hour. Bright daylight signaled the need for alertness and elevated metabolic demands, while darkness triggered the onset of rest and sleep.

While modern life is arguably safer and more bountiful than hunter-gatherer times - primarily thanks to the invention of electricity - our reliance upon artificial lighting and heavy screen use may be making us sick.

Most of us spend too much time indoors, and the lack of daylight wreaks havoc on our physiology. Combined with light pollution at night, this results in a weaker circadian rhythm, sleep problems, mood disorders, and even more severe health complications such as cardiovascular disease and cancer.

Circadian Regulation

The nuances of natural daylight are surprisingly difficult to replicate indoors. While overhead bulbs may be perceived as daylight-mimicking, the specific combination of wavelengths rarely comes close to that experienced under sunlight or even a gloomy overcast day. One factor is that the bulbs cannot account for the subtleties in how the sun moves across the sky. Another area is that the bulbs can't account for the spatial distribution and even the polarization of sunlight.

On a cloudy day, the illuminance is about 3,000 lux, with levels soaring to well above 20,000 lux on bright, sunny days. There's a natural shift from shorter, blue wavelengths of light during the first part of the day to longer, amber wavelengths as twilight approaches. Exposure to both daily helps keep our internal clock synchronized with the environment.

Time-keeping genes are found in almost every cell of the body, and these dictate when cells perform their various functions. They run on a clock that is slightly longer than 24 hours and rely on signals from a central clock in the brain (the suprachiasmatic nucleus - SCN) to interpret light entering the eye as a time cue to pull us back to the 24-hour solar day. This also starts an internal clock to allow sleep to set in about 16 hours later, aided by the release of the hormone melatonin.

At the equator, 12 hours of daylight precede 12 hours of darkness every single day of the year. In this environment, the brain knows what to expect and can predict the right time for daily wake and sleep. As we move closer to the poles and experience shorter day lengths in the winter, the sun rises after we've already started our workday and sets well before we're ready to sleep.

The delayed sunrise can alter our internal "stopwatch," cutting sleep short at both ends, leaving us sleep-deprived and tired during the daytime.

Individuals living in New York City will face greater day length extremes than those in Los Angeles, but not everyone will necessarily suffer as a result. Genetic factors predispose some people to winter depression (seasonal affective disorder - SAD,) while lifestyle factors also play a role.

The best way to stay synchronized and promote a desired bedtime is to spend as much time outside during the first half of the day as possible. Sunlight-mimicking lamps can be used for short winter days and make a significant difference when used immediately upon waking. Focusing on smart lighting and regular breaks outside are essential for those working in spaces with little natural light.

After maximal exposure to light during the day, support melatonin secretion at night by keeping the lights low beginning three hours before bedtime and throughout the night. Experts recommend just 10 lux in the evening (equivalent to a dim, orange-toned table lamp) followed by less than 1 lux at night (which is almost total darkness.) Melatonin is a potent antioxidant that prevents cancer, lowers blood pressure, and helps us fall and stay asleep. Consider using orange or red-tinted, blue light-blocking glasses if you want to take extra care to block melatonin-suppressing blue light wavelengths from screens at night.

Over 99% of the US population is exposed to light at night, and almost a quarter of the planet's land mass experiences light pollution. Combined with blue light emitting devices such as cellphones and laptops, this light exposure delays bedtime and can promote more nighttime awakenings. This leads to next-day sleepiness, trouble focusing, and for some, an increased risk of depression.

Vitamin D

The importance of daylight stretches beyond its circadian effects on the brain. Skin must be regularly exposed to the sun for it to synthesize vitamin D, which is essential for healthy bones, mood, and immunity; it can be sourced from foods such as oily fish, eggs, and dairy. Some studies comparing the sustainability of bioactive vitamin D when acquired through diet versus sunlight have shown that sunlight-sourced vitamin D levels remain elevated for much longer than when it is ingested. There are also specific vitamin D metabolites that are exclusively produced by sunlight.

Vitamin D synthesis begins when ultraviolet (UVB) light interacts with the skin to produce vitamin D3. D3 from the skin, along with D2 and D3 from the diet, are metabolized by the liver into 25-hydroxyvitamin D (25(OH)D), also known as calcidiol, which is biologically inert. It's not until this 25(OH)D passes through the kidneys that it is activated to 1,25(OH)2D (calcitriol), which promotes calcium and phosphorus absorption from the gut. This uptake is essential for bone health and a wide array of metabolic functions.

For example, for those with fair skin living in the latitude of New York City, just 15 minutes of exposure to midday sun (11 am - 3 pm) on the arms and face about three times per week is considered plenty for the skin to synthesize enough vitamin D. Those with darker skin tones need longer, and the rate of synthesis can also decrease with age.

The "healthy" target serum level of vitamin D is widely debated. Some recommend 20 ng/mL, while others suggest 30 ng/mL is more appropriate. There has been some controversy over the years regarding the extent of vitamin D deficiencies in the population, fueled mainly by the supplement industry and not always supported by rigorous science.

The body can store vitamin D, so adequate exposure from spring through fall may be sufficient to carry some people through the winter months. However, there are differences between individuals, with some people thought to be better protected than others.

Those at risk of becoming truly deficient include people with darker skin tones living at higher latitudes, those with kidney disorders, shift workers, immobility issues, seniors, and those eating poor-quality diets. Some evidence shows that diets rich in fructose can impair the enzymes that activate vitamin D once it is in the body. Therefore, as well as sunlight exposure, being mindful of sugar intake is essential.

Vitamin D is vital for serotonin and melatonin regulation, which are integral for healthy mood and sleep. Individuals with a vitamin D deficiency are more likely to report poor quality sleep, daytime sleepiness, and even a sleep disorder.

Vitamin D may also be a powerful preventative against myopia (nearsightedness) by regulating the eye's ciliary muscles. These muscles are responsible for adjusting the shape of our lens according to how far away we are from the object we are trying to focus on. Too much time spent looking at objects directly in front of us (such as screens) can also increase the risk of myopia. Hence, time spent outdoors offers vitamin D and ample opportunities to practice long-distance focus.

Mindset

As well as making the effort to get outside and use light strategically, there is good reason to embrace a positive mindset during the winter months, Scandinavian style. While hygge alone cannot cure seasonal depression, at least one survey study has shown that those who felt positively about winter also reported greater life satisfaction and mood during the coldest months. Gratitude is now widely reported to improve well-being, so shifting your focus to any positives associated with the colder months can offer support through the shorter days.

Our take:

Most of us spend a significant proportion of our time indoors, but finding opportunities to take breaks outside benefits physical, mental, and emotional health. Starting the day outdoors with physical activity often has many other advantages, especially if spent in nature. You do not have to be outdoors for every workout. But make a conscious effort to enjoy exercise outdoors when you can. Even a short walk has many benefits for cardiovascular and mental health.

Add small opportunities for more daylight exposure by walking instead of driving, parking further away from the grocery store, and taking a few extra steps. Take phone calls outside, or even push for screened meetings to be at a minimum. Take lunch outside, and have a raincoat and appropriate shoes on hand so that weather is never an excuse. For a more significant challenge, consider trying a snow sport or cold water swimming to maximize the benefits of wintertime outdoors.

Will this benefit you?

Prioritizing time outdoors during the winter is valuable. Any family, friends, or colleagues you can convince to join will gain from these endevours, whether it's a walk outside or the cozy aftermath of a walk well-trodd.

Instead of focusing so much on what isn't available during the winter, put your energy into challenging yourself to find new ways of enjoying the cold. As stated by many great outdoors folks before us, there is no such thing as bad weather - only bad clothing. Get prepared with a few essentials, and you might surprise yourself with how much you enjoy it.

Still curious to try it? Here's what to look out for if you do:

Although the sun is much weaker during the winter at higher latitudes, there is always an increased risk of skin cancer with extended sun exposure. If you are concerned about sunscreen use and vitamin D, speak with your doctor and see what protocol is best for you. Supplements might be helpful if you're at a higher risk of becoming deficient.

As science continues to explore the extent to which modern lifestyles and workplaces impact circadian rhythms through reduced daylight exposure, urban development will likely change to better support health. The challenge for office design will be maximizing the amount of biologically-active light that can enter the space while minimizing glare. Buildings with atria, skylights, and courtyards can promote more light exposure, as well as policies that normalize time outside away from the desk. Be mindful of the changes you can make to support your circadian rhythm, and reap the benefits for years to come.

References:

The relevance of daylight for humans: https://pubmed.ncbi.nlm.nih.gov/33129807/

Daylight savings time problem explained: https://www.washingtonpost.com/wellness/interactive/2022/permanent-standard-time-body-health-benefits/

Physical activity indoors vs outdoors: https://pubmed.ncbi.nlm.nih.gov/21291246/

Urban environments and light: https://pubmed.ncbi.nlm.nih.gov/35023979/

Controversies surrounding vitamin D: https://pubmed.ncbi.nlm.nih.gov/30641860/\

Am I getting enough vitamin D?: https://www.nytimes.com/2018/02/16/well/live/do-i-get-enough-vitamin-d-in-the-winter.html

Vitamin D and immune function: https://pubmed.ncbi.nlm.nih.gov/23857223/

Vitamin D, serotonin, and melatonin: https://pubmed.ncbi.nlm.nih.gov/32987320/

Vitamin D biosynthesis: https://www.nature.com/articles/s12276-018-0038-9

Vitamin D controversy: https://www.nytimes.com/2018/08/18/business/vitamin-d-michael-holick.html

Fructose and vitamin D deficiency: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0093611