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There's a New Scientific Explanation for Seasonal Affective Disorder

As the days get darker, a protein known as SERT goes into overdrive, making you feel crappy.
Image: Brian Donovan/Flickr

Even as someone that loves ice, slop, rain, snow, and cold, it's hard not to look at a forecast like this and feel a pinge of grey emptiness:

That's winter around here, basically. Every now and again there will be a proper cold snap and one of those days will flip to snow, and we'll get buried underneath two feet of the heaviest, wettest slop nature is capable of. It would seem these are conditions uniquely primed for seasonal affective disorder, an extremely common condition considered to be a recurrent form of major depressive disorder.

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How those endless days of soaking-wet bleakness actually affect the brain has remained something of a mystery. The common assumption is that it's related to light; as daylight hours disappear, our circadian rhythms fall out of order. So: we feel bad.

In a study being presented this week at the European College of Neuropsychopharmacology Congress, a team of researchers describe a "dial" of sorts that the brain uses to adjust its serotonin levels in response to seasonal changes. What they found is that, as sunlight diminishes during the winter months, so do serotonin levels. Some people experience this more than others.

Using Positron Emission Tomography (PET), the researchers, led by Danish neurobiologist Brenda McMahon, monitored the brain activity of 11 patients diagnosed with SAD and 23 without. Comparing summer to winter, they found significant differences in the quantities of a protein used in the brain, known as SERT, to transport the neurotransmitter serotonin.

"We find that SAD patients experience a significantly larger seasonal SERT fluctuation compared to their healthy counterparts," the study concludes. "We were not able to reproduce previous findings of a similar up-regulation during winter in healthy subjects and we speculate that this is due to a careful selection of individuals completely void of season related symptoms. Our data suggests that seasonally provoked depression is linked to seasonal SERT changes."

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Serotonin is the neurotransmitter most closely associated with mood; most antidepressant drugs act by inhibiting the processes the brain uses to clear serotonin from synapses. So: more serotonin remains, people feel better.

SERT's job is basically to move serotonin around. The more SERT, the less serotonin is allowed to hang out within a given synapse. McMahon et al found that, among SAD patients, SERT levels spiked as daylight hours diminished. Healthy participants showed no change in SERT levels, while SAD patients experienced an average fluctuation of 5 percent.

That's a small number for a small sample size, but serotonin has been implicated in SAD in many other studies. In particular, research has found lower levels of the amino acid tryptophan, a precursor to serotonin. In one classic study, researchers found tryptophan levels spiking in SAD patients around May and June, with levels falling in late-summer and early-autumn.

There are literal volumes of SAD/serotonin research. For another example, take a review study released in 2013 (a study of other studies), finding simply, "It has been concluded from the whole study that serotonin, an endogenous chemical plays an important role in [SAD]." The success of light-therapy and serotonin-specific drugs in treating SAD offers yet more conformation.

"The least offensive, most natural, and most studied treatment of SAD is light therapy," the 2013 review offers. "The original theory behind light therapy was that it would cause a normalization of the phase-shift delay in SAD. It was then thought to increase the photoperiod in winter in those with SAD. It has also been used to abolish the production of melatonin by the pineal gland."

The pineal gland, which produces melatonin by stealing away serotonin, goes into overdrive in the presence of darkness. More melatonin, less serotonin. Things feel bad.

It's worth emphasizing that SAD isn't a thing you just have to deal with. It's common and often viewed as a bit jokey, but depression, seasonal or not, is often devastating. It might rain three inches in the next three days, but, with something as simple as sunlight-mimicking lights, it doesn't have to feel like that on the inside.