If your mood sinks and energy fades specifically in winter, it might not just be "in your head." Seasonal Affective Disorder (SAD) is a recognized condition where changes in sunlight actually alter brain chemistry. Here's the mechanism behind it and how to cope.
Seasonal Affective Disorder is a type of major depressive disorder recognized by the US National Institute of Mental Health and others, characterized by a distinct seasonal pattern: symptoms appear from late fall through winter and improve on their own by spring. It's known to occur more frequently the farther you live from the equator, at higher latitudes.
Serotonin is a key neurotransmitter that regulates mood. Less time exposed to sunlight lowers serotonin levels in the brain, which is considered one of the core drivers of low mood. Vitamin D, which tends to run low in winter, is also thought to support serotonin activity, so reduced sunlight can affect serotonin through two paths at once.
Unlike serotonin, melatonin — which regulates the sleep-wake cycle — actually rises when sunlight is scarce. In people with SAD, the melatonin secretion cycle can lengthen or shift later, so melatonin that should only appear at night extends into daytime hours, contributing to daytime drowsiness, mental fog, and lethargy.
Both serotonin and melatonin help maintain the body's circadian rhythm in line with changing day and night length. In people with SAD, the balance between these two breaks down, so the body can no longer properly adjust to seasonal changes in day length — leading to changes across sleep, mood, and behavior.
The primary treatment is light therapy — exposure to bright artificial light for a set time each day. It works by artificially supplementing the natural light that's lacking, helping restore the balance between serotonin and melatonin. Increasing daytime outdoor activity, and pairing this with counseling or medication when needed, can also help.
Even brief outdoor sun exposure during daytime hours on a clear day can help. Checking sky condition and sunrise/sunset times in sanCheck helps you make the most of winter's shorter daylight hours.