Seasonal depression typically lasts about 4 to 5 months per year. For the most common form, winter-pattern seasonal affective disorder (SAD), symptoms begin in late fall or early winter and lift during spring or summer. The episode is predictable, recurring around the same months each year, and for most people it resolves on its own once daylight hours increase.
When Symptoms Start and Stop
Most people with winter-pattern SAD notice symptoms creeping in during October or November. The worst stretch tends to fall in January and February, when daylight is shortest. By March or April, symptoms begin to ease, and most people feel fully recovered by May. That means a typical episode runs roughly five months, though the exact window depends on where you live. If you’re in Alaska or northern Scandinavia, winter darkness arrives earlier and lingers longer, which can extend the depressive episode on both ends. If you’re in a mid-latitude state like Virginia or Oregon, the window may be slightly shorter.
Geography matters more than you might expect. A 2025 meta-analysis found that for every one degree of latitude farther from the equator, SAD prevalence rises measurably. In Alaska, nearly 9% of the population meets criteria for SAD, compared to about 6% across the broader U.S. population. An additional 14% of Americans experience a milder version called subsyndromal seasonal depression, sometimes referred to as the “winter blues.”
Winter Blues vs. Clinical SAD
Not every dip in mood during winter qualifies as SAD. The National Institute of Mental Health draws a useful line between the two. If you feel a bit low energy and have some trouble sleeping but can still manage work, school, and daily life, and the feeling lasts less than two weeks, that’s likely the winter blues. It’s common and usually passes without intervention.
Clinical SAD is more disruptive. Symptoms persist for more than two weeks and include social withdrawal, oversleeping, weight gain, and intense cravings for sugary or starchy foods. People with SAD often describe feeling like they physically cannot get through the day, not just that they’d rather stay in bed. A formal diagnosis requires that this seasonal pattern has occurred for at least two consecutive years, with full or near-full remission during the opposite season, and that these seasonal episodes outnumber any non-seasonal depressive episodes over your lifetime.
Why It Happens When It Does
The timing of seasonal depression is driven by your body’s internal clock falling out of sync with the actual day-night cycle. As days get shorter in fall, your brain starts producing melatonin (the hormone that signals sleep) earlier in the evening and keeps producing it later into the morning. For most people, this shift is minor. For people with SAD, the shift is exaggerated, creating a mismatch between when their body thinks it should be sleeping and when they actually need to be awake and functioning.
Research from clinical studies shows that the typical SAD patient’s internal clock runs late relative to their sleep schedule. A smaller subgroup has the opposite problem, with their clock running too early. In both cases, the core issue is the same: circadian misalignment. When daylight returns in spring and resets the clock, symptoms lift. This is also why light therapy works. Bright light exposure in the morning pushes the internal clock earlier, correcting the delay that drives symptoms in most people.
Vitamin D may play a supporting role. Levels of vitamin D drop during winter months due to reduced sun exposure, and research consistently links low vitamin D to higher depression severity. In a population study of over 1,200 older adults, vitamin D levels were 14% lower in those with major depression compared to controls, even after adjusting for age, sex, smoking, and other health conditions. Whether supplementing vitamin D shortens a seasonal episode is less clear, but the connection between low levels and worse mood is well established.
Summer-Pattern SAD
A less common form of seasonal depression follows the opposite calendar. Summer-pattern SAD begins in spring and lasts through summer, lifting in fall. It also runs about 4 to 5 months. The symptoms tend to look different from winter SAD: insomnia rather than oversleeping, loss of appetite rather than carb cravings, agitation, and anxiety. The triggers are thought to involve excess heat and longer daylight disrupting sleep rather than too little light.
How Quickly Treatment Helps
If you use a light therapy box (the standard first-line approach for winter SAD), improvements often appear within the first week. Full effect, though, can take 3 to 6 weeks of consistent daily use, typically 20 to 30 minutes each morning. Light therapy doesn’t shorten the season, but it can reduce symptom severity enough that you function well throughout it. Most people continue using the light box daily until spring, then taper off as natural daylight takes over.
For people whose symptoms are more severe, antidepressants are sometimes started before the usual onset window, often in early fall, and continued a few weeks past when symptoms would normally resolve. This preventive approach can blunt the episode before it fully develops. The Mayo Clinic notes that providers often recommend continuing medication beyond the typical resolution point to avoid a relapse during the tail end of the season.
Recurrence Over the Years
SAD is, by definition, a recurring condition. The diagnostic criteria require a pattern that repeats year after year, with seasonal episodes outnumbering non-seasonal ones across your lifetime. In practical terms, if you had SAD last winter, you will very likely have it again next winter. Some people experience it every year for decades. Others find that it becomes less predictable over time or shifts into non-seasonal depression.
The predictability of SAD is actually one of its few advantages. Because you know roughly when symptoms will arrive, you can prepare. Starting light therapy or medication in early fall, before symptoms hit, is more effective than waiting until you’re already deep in an episode. Building outdoor time, exercise, and social commitments into your fall routine can also reduce the severity of what’s coming, even if it won’t prevent the episode entirely.