Seasonal mood, it's not just SAD
Seasonal Affective Disorder was formally named forty years ago. The clinical picture is specific: major depressive episodes that begin in fall or winter and remit in spring, with hypersomnia, carbohydrate cravings, fatigue, and social withdrawal as characteristic features. Prevalence estimates put it at 1–6% of the general population, rising significantly at higher latitudes.
But the full seasonal picture is much wider than the clinical diagnosis.
Sub-threshold seasonality
Researchers distinguish between SAD, where seasonal variation causes clinically significant impairment, and "sub-threshold SAD" or simply mood seasonality, where energy, sleep, appetite, and sociability shift with the seasons without crossing the threshold for a formal diagnosis. Studies suggest 10–30% of people with major depression or bipolar disorder experience meaningful seasonal mood shifts. In general populations, the number of people who notice some seasonal variation in mood and energy is far higher.
This matters because sub-threshold doesn't mean inconsequential. A sustained winter energy dip that doesn't meet diagnostic criteria can still erode motivation, strain relationships, and make difficult periods harder. It just doesn't get a treatment plan.
The biology behind it
Serotonin levels vary across seasons, higher in summer and lower in winter, driven largely by sunlight. The serotonin transporter that removes serotonin from the synapse is more active in winter, meaning less serotonin stays in circulation. Melatonin production extends into longer winter nights, which shifts the timing of sleep pressure and energy levels. These are not subtle effects; they're detectable in population-level data on mood, activity, and social behavior.
Higher latitude amplifies everything. A meta-analysis published in 2025 confirmed that SAD prevalence increases with distance from the equator. Whether that's driven by shorter winter day length, colder temperatures, reduced outdoor activity, or all three is still debated.
What a year of data shows you
One of the things a yearly mood heatmap in Mood surfaces without any interpretation is seasonal shape. Most people who've tracked through a full year can point to a visible band, typically November through February in the Northern Hemisphere, where the colors shift. Sometimes it's subtle. Sometimes it's unmistakable.
Seeing it in your own data is different from knowing it abstractly. It removes the self-doubt that often accompanies seasonal lows: am I just unmotivated? Is this really that bad? A visible pattern is an answer.
Light therapy turns forty too
The treatment with the strongest evidence for seasonal mood variation is bright light therapy, 10,000 lux for 20–30 minutes each morning. A 2024 meta-analysis confirmed its efficacy not just for SAD but for non-seasonal depression as well. It's cheap, has minimal side effects, and works faster than most antidepressants for the right presentation.
What doesn't work: windows on overcast days, office lighting, and most indoor environments. The target lux level is roughly 20 times brighter than typical indoor light.
Seasonal mood variation doesn't need a diagnosis to be worth paying attention to.