Everyone gets a little less energetic as daylight shrinks, but for some people winter brings a real, recurring drop in mood that others never experience. One of the earliest and most replicated findings on this, a study on the serotonin transporter gene and seasonal affective disorder, found that a specific version of a gene controlling serotonin transport was more common in people with seasonal depression and linked to higher seasonality scores overall. Genetics doesn’t cause winter mood changes by itself, but it can set how sensitive your brain’s chemistry is to shrinking daylight.
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How the Serotonin Transporter Gene Leaves Some Brains More Exposed in Winter
The serotonin transporter, built using instructions from the SLC6A4 gene, controls how much serotonin is available at the junctions between brain cells. A stretch of DNA near this gene, called 5-HTTLPR, comes in a shorter and a longer version, and the shorter version tends to produce less transporter activity.
What’s interesting is what happens seasonally. In healthy people, the brain naturally reduces serotonin transporter levels during winter, which helps preserve more usable serotonin when daylight-driven production drops. A brain imaging study using PET scans found that people with seasonal affective disorder, especially women carrying the short version of this gene, failed to make that seasonal adjustment the way healthy people do. In other words, their brains kept clearing away serotonin at the same rate even as the raw material for making it became scarcer, leaving less available exactly when it was needed most.
Why the OPN4 “Melanopsin” Gene Changes How Much Light You Actually Need
Vision isn’t the only thing your eyes do. A separate set of light-sensing cells in the retina, powered by a light-sensitive protein called melanopsin, feeds directly into the brain circuits that regulate mood, alertness, and the body’s internal clock, independent of the circuits used for seeing.
The gene that builds this protein is OPN4, and a specific variant of it has been directly studied in seasonal affective disorder. A case-control genetic study found that people carrying two copies of a particular OPN4 variant were over five times more likely to have seasonal affective disorder than those without it. The leading theory is that this variant raises the amount of light needed to send a normal “it’s daytime” signal to the brain, so on a dim winter afternoon that would barely register for one person, someone with this variant’s brain may be getting something closer to a dusk or nighttime signal.
The Role of Circadian Clock Genes Like CRY2 in Seasonal Mood Shifts
A third layer involves the genes that run your internal 24-hour clock. One of these, CRY2, has repeatedly turned up in research on seasonal mood changes.
A recent study analyzing clock gene variants and seasonality symptoms confirmed earlier links between CRY2 and seasonal mood shifts, alongside related genes like PER2 and PER3, and found that these effects often work through their influence on a person’s natural sleep-wake timing preference. Separate modeling work has suggested that certain CRY2 variants are tied to a bigger seasonal drop in the amplitude of circadian gene activity during winter, meaning the whole internal clock system swings more sluggishly when daylight is scarce. A blunted, sluggish clock signal in winter is a plausible mechanism for the kind of low energy and low mood that comes back every year around the same time.
Genetics Sets Sensitivity, Not Destiny
Carrying any of these variants doesn’t guarantee seasonal mood changes, and plenty of people without them still notice a winter dip. Latitude, indoor lifestyle, existing depression or anxiety, vitamin D status, and sleep habits all interact with this genetic sensitivity. Genetics is best understood as a dial that sets how much a given amount of lost daylight actually affects your brain chemistry, not a fixed sentence.
If winter reliably hits you harder than it hits the people around you, it may be worth understanding whether your own serotonin transport and light-sensing genes are part of why. A report covering the serotonin and melatonin pathway can map these specific variants against your own DNA.
Frequently Asked Questions
Is seasonal mood change actually genetic?
Research has linked specific variants in genes involved in serotonin transport, light sensing, and circadian rhythm to seasonal affective disorder, suggesting genetics can shape how sensitive a person’s mood is to shrinking daylight, though it works alongside lifestyle and environmental factors.
What does the OPN4 melanopsin gene have to do with mood?
OPN4 builds a light-sensing protein used by retinal cells that feed directly into the brain’s mood and circadian regulation circuits. Certain variants may require more light to send a normal “daytime” signal, which could make dim winter days feel biologically more like nighttime.
Why do some people’s brains fail to adjust to less winter serotonin?
Brain imaging research has found that people with seasonal affective disorder, particularly those carrying a shorter version of the serotonin transporter gene, don’t reduce their serotonin transporter activity in winter the way healthy people typically do, leaving less serotonin available when it’s needed most.
