The Winter Solstice Is Here. How Dark Days Affect the Human Body

Our bodies are so ready to celebrate the rebirth of the Sun

Who Gary Stix edited by Dean Visser

The moment when the Earth’s Northern Hemisphere tilts furthest from the sun—the December solstice, on the 21st of this year—is not just a mark on the calendar. It is also defined by the way our bodies react to the event. The dimming of our daily natural light leading up to the winter solstice produces some remarkable physiological changes.

These changes correspond to circadian rhythms. The word circadian comes from the Latin “about diemmeans “about a day”. It describes how animals, plants, fungi and bacteria react to environmental cues, including light input, on a daily and seasonal basis.

Sofia Axelrod is a chronobiologist at Rockefeller University who studies circadian rhythms and their impact on physiology and behavior. His research on circadian rhythms, sleep and longevity in the laboratory of Nobelist Michael Young made him an ideal candidate to ask how the solstice and the dark days before it affect fruit flies (the animals he began studying). ) to humans.

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(Following is an edited transcript of the interview.)

What happens to our body rhythms at higher latitudes during the shortest day of the year?

Our body rhythms are set by the lights. Your internal clock can become out of sync in real time, for example, when you travel from east to west and when light exposure changes. This also happens with the shortest day of the year, because the light exposure in summer is four to eight hours earlier than in winter. In Berlin, where I’m from, the sun rises at 3:45 am on June 21st and at 8:15 am on December 21st. So at this point we don’t get the signal of the day until the time we have to get up to go to school. or work, which feels overwhelming and unhealthy for our circadian rhythm. And so you have this delayed onset of the circadian stimulus, which has to tell your body through the eyes and a specialized brain structure that it’s time to start activating (transcribing and translating) a set of essentially similar clock genes. a secretary of all cells and tell other cells when to do what.

In addition, we don’t get enough sunlight throughout the day either, because it gets dark very early: 15:56 in Berlin, to stay with that example. It’s the dark hours before you need to fall asleep, which can have adverse effects on people’s mood, energy levels and sleep, and in older people with dementia it can worsen ‘sunset’, leading to confusion, agitation and sleep disturbances.

How do people with sleep disorders react?

What you see in the winter is that people, if left to themselves, get out of bed later, simply because they don’t get that light stimulation to start the day. Also, due to the influence of indoor lighting in our modern society, there are big changes in our sleep-wake duration.

We all experience this. It is very difficult to get out of bed when it is pitch black, and conversely, in the summer, it is very difficult to sleep when you are in a very bright room and receiving a lot of sunlight at 4 in the morning. Is this all healthy? Isn’t it healthy? Nothing I’ve just described suggests it.

But light sensitivity varies between people. Generally, there is no problem with this, unless it interferes with your ability to function in some way. Many people have trouble getting out of bed without light, which makes it difficult for them to function during the day. Then it becomes difficult, because there is the phenomenon of the lack of sunlight in the winter, which causes seasonal affective disorder, a circadian disruption that makes people feel very weak. And that is a real thing caused by a complete lack of light.

It’s not just the length of the shortest day of the year that counts when the sun rises. It is also the general light level at higher latitudes. In New York City, where I am, some days it is very dark. The light level never reaches the amount or dose needed to teach your circadian rhythm. If that’s the state of your environment for a long time, that causes a major disruption in the circadian clock, essentially no longer doing its job of organizing your cellular functions. And one result of that is depression.

Is an effect like depression particularly acute in the immediate period around the winter solstice?

Yes, especially for people who get up early and basically go to work in the dark, sit in a windowless office with indoor lighting that doesn’t provide circadian stimulation, and come home again when it gets dark. Basically, they have spent, potentially, weeks in total circadian darkness. And, of course, all this is most acute around the solstice, because it is the shortest day of the year, and then it improves again. And people in some cultures describe this as a revival, and it’s really a revival of the circadian clock.

Are there other effects besides depression? How does the solstice affect something like resistance to infection?

All cells have a circadian rhythm. If you don’t have enough light because it’s winter, the immune system gets damp. At certain times of the day, the cells you need to fight infections—T cells, macrophages—stop making. A less well-functioning immune system and less resistance to infection.

There has been talk of removing daylight saving time, including from the incoming presidential administration. Would that be good?

Twice a year we change our bodies with daylight savings, which causes jet lag. It may not be a big deal to any one person, but statistically it is a big deal on a general population level. Heart attacks and traffic accidents jump the next day. It’s just a useless thing that we are subjecting our whole country to, and we should get rid of it.