I used to think my ‘winter pouch’ was just an instinctual response to the cold.
I’d hunker down indoors like a Hobbit, eat some more carbs to put on an extra insulating layer of blubber and save my energy for spring.
But over the years I came to realize this pattern was less a response to cold than to light.
It was the deadly darkening at 4pm and the endless nights that followed that really put my physiology in power saver mode.
By late winter, the results were obvious. A little pudge, a foggy brain, a darkened mood, a metabolism that seemed incapable of kicking it into higher gear.
Then spring would come, and it would all go away. I could feel the cobwebs burning off in that first stretch of bright days, and I could see the pudge melt away in the weeks to follow.
By midsummer, my weight was down, my energy up, my outlook on life entirely positive. And then it would happen all over again the following winter.
For a long time, I thought of this pattern as mostly circumstantial. I figured that the short days and cold temperatures made it difficult to do too much outside, so I tended to stay indoors and be more sedentary.
‘By late winter, the results were obvious. A little pudge, a foggy brain, a darkened mood, a metabolism that seemed incapable of kicking it into higher gear,’ writes Jacobsen
Cold temperatures make it difficult to do much outside – but making the effort pays dividends
But, while researching my new book, In Defense of Sunlight, I’ve learned that light itself has a profound and wildly underappreciated impact on metabolism.
Sun hitting skin can change everything from the amount of fat you store to the amount of energy you burn. It impacts the health of your heart, brain and immune system. Understanding why requires a reconsideration of what skin is and what it’s trying to do.
We tend to think of the skin as mere packaging, a way to wrap the important parts of the body and protect them from the outer world. But we now have decades of studies showing it is much more than that.
The skin is our largest organ and one of the primary generators of hormones and other signaling molecules used for cellular communication. Through the nervous and endocrine systems, it’s linked to every other organ in the body, including the brain.
It’s also loaded with opsins, the same light-sensing proteins found in our eyes. It’s an observatory. As the interface with the outer world, it monitors the environment all day long and sends a steady stream of reports to the rest of the body.
When sufficient sunlight falls on the skin, those reports lean in favor of a high-activity mode. Metabolism rises, fat stores get liquidated, blood vessels dilate, cognition increases, inflammation falls, endorphins flood the brain and our mitochondria raise their game.
We get friskier when the sun shines.
And we get thinner.
Sun hitting skin can change everything from the amount of fat you store to the amount of energy you burn
When sufficient sunlight falls on the skin, metabolism rises, fat stores get liquidated and we get thinner
In the lab, mice fed high-fat chow and exposed to regular doses of modest UV light (equivalent to about 30 minutes of summer sunshine) were much less likely to gain weight than mice who didn’t get the light treatments.
After 12 weeks, they had 23 percent less fat and just half the rate of atherosclerosis. In the human world, rates of obesity, diabetes and insulin resistance are lower in summer than in winter.
But when the light disappears, the opposite happens. Humans don’t hibernate like bears, but shield us from light long enough, and we descend into a kind of power saver mode, storing calories and eking out a depressed existence through the perceived ‘biological winter’ until spring returns.
In ancient times, those were useful adaptations for surviving winter, when food might be scarce for long stretches.
The problem today is that most of us get so little exposure to natural light. The average person spends barely an hour outside per day, a staggering change from our ancient past. The rest of our time is spent in artificial lighting, which produces only a tiny fraction of the light of the sun, and lacks many of the key wavelengths altogether.
That sends screwy signals to our body and leaves us in permanent circadian confusion.
The result, many scientists believe, is an epidemic of poor health. A growing body of evidence now links sun deficiency with a staggering number of diseases, from cardiovascular illness and diabetes to dementia, depression and autoimmune diseases such as multiple sclerosis, as well as some internal cancers.
People who get more light consistently live longer than people who get little.
Proving that such associations are causative is always difficult with any population-wide study, but in this case, there’s no sign that sun avoidance improves overall health, so we have every reason to assume that it’s beneficial to follow our evolutionary norm of spending a lot of the day outside, unless some very good research comes along proving otherwise.
People who get more light consistently live longer than people who get little. (Pictured: Jacobsen snorkeling)
What about skin cancer? It’s an important consideration, but the biggest risk factor for melanoma is not how much sun you get but how fair your skin is.
People with very fair skin have a substantially elevated risk of melanoma, and that goes up even more if they have red hair or many moles. They do indeed need to be extremely cautious about sun exposure.
For people with darker skin, the risks are much smaller.
For most people, the benefits of sunlight greatly outweigh the risks. Skin cancer is responsible for one in 500 deaths worldwide. Cardiovascular disease is responsible for one in three. Anything that can lower that is a boon to public health.
We can see that by looking at Australia. As one of the sunniest countries in the world, with a predominantly fair-skinned population adapted to the soft lighting of the British Isles, you’d expect Australia to have sky-high rates of skin cancer, and it does – the highest in the world.
It also has one of the highest rates of mortality from melanoma, more than twice that of the US.
If all that sunlight was a major burden on public health, you’d expect to see it in the overall life expectancy, but on that all-important metric Australians leave their mates in the US and other English-speaking countries well behind, thanks to lower rates of overall cancers, cardiovascular diseases, and respiratory infections – all the conditions that seem to be improved by sunlight.
They actually have some of the best life expectancy in the world. And according to the recent viral news reports, they may be sexier as well. More sun, more fun, fewer clothes, no worries.
But that doesn’t mean we all need to move to Melbourne to achieve our own Australia Effect – a trend that went viral on TikTok last year. There are lots of ways to spur the body out of its biological darkness.
The easiest is to simply go outside, the earlier the better. Natural light can be a hundred times brighter than the artificial light indoors; as those photons find your eyes and your skin, your body will respond.
Morning is a great time to do this without risking any damage from UV radiation, and shade is great anytime.
For most people, the benefits of sunlight greatly outweigh the risks, argues Jacobsen
Go outside, the earlier the better – natural light will work its magic and your body will respond
To make vitamin D and certain other beneficial compounds, get some skin in the sun midday (not your face, which tends to get too much), always being careful not to burn, or even to get anywhere close to it.
In many colder places, doing that becomes virtually impossible in the winter months. SAD lamps can help a little. Saunas also seem to deliver some similar benefits.
And soon, more help will arrive with a new generation of physician-approved UV lamps (which offer a much safer spectrum of light than tanning beds, which can contribute to melanoma) designed to produce vitamin D and other compounds in the comfort of the home.
In the meantime, now is the perfect time to celebrate the sun’s ascendency and take stock of all its health benefits. Working with it, instead of against it, can help to put the winter blues firmly behind and get you on track for your best summer body.
Rowan Jacobsen is the author of In Defense of Sunlight: The Surprising Science of Sun Exposure (Scribner, 2026).
