Seasonal affective disorder is a form of major depression that follows a seasonal pattern, typically beginning in fall, deepening through winter, and lifting in spring. Roughly 5% of US adults experience it, and it is about four times more common in women than in men (American Psychiatric Association). The part that matters right now is the timing: it is far easier to prevent than to climb out of, and the window for prevention is open in September.
Most people call in January. By then the season has been working on them for two months, their sleep and energy are already compromised, and the effort required to start anything is at its lowest point of the year. That is the worst moment to begin, and it is when almost everyone begins.
The pattern is the point
Seasonal depression is unusual among mental health conditions in that it announces itself in advance. If it happened last year, and the year before, you have something most people never get: a reliable forecast.
That changes what is possible. With most depression you are responding to something already underway. With a seasonal pattern you can put things in place while you still have the capacity to put things in place. The version of you reading this in September has energy the version of you in December will not have.
So the useful question in September is not “am I depressed.” You probably are not, yet. It is “what happened to me last October, and what would I want already handled before it happens again.”
What the research says about what lasts
Light therapy is the intervention most people have heard of, and it works. During the season you use it, it performs about as well as therapy.
The difference shows up later. In a randomized trial of 177 adults with recurrent seasonal depression, participants received either six weeks of light therapy or six weeks of cognitive behavioral therapy adapted for seasonal depression, then were followed across the next two winters. One winter later the two groups looked similar, with recurrence around 25 to 29% in both. Two winters later, recurrence was 45.6% for light therapy and 27.3% for CBT-SAD (Rohan et al., American Journal of Psychiatry, 2016).
Roughly, light therapy treats this winter. Therapy adapted for seasonal depression appears to change how the next several winters go.
That is not an argument against light therapy, which is effective, accessible, and easy to start. It is an argument against treating it as the whole plan.
Why the beliefs matter more than they sound like they should
The mechanism researchers identified for that durability advantage was change in seasonal beliefs. Not light exposure, not mood ratings during treatment, but what people came to think about winter.
That sounds soft until you notice how the beliefs operate. Winter ruins me. There’s no point starting anything until spring. I just have to get through it. Each one is a prediction, and each one licenses withdrawal. You cancel plans, stop exercising, stop seeing people, and the withdrawal produces exactly the season you predicted, which confirms the belief for next year.
The loop runs on its own once it starts. Interrupting it in September, when nothing has happened yet and the prediction has not been made, is considerably easier than interrupting it in January from inside it.
Yes, this happens here
The most common reaction I get to any of this in the South Bay is that we live in Southern California.
Prevalence does follow latitude. Estimates run around 1.5% in Florida and close to 10% in New Hampshire, so this is genuinely less common here than in Vermont. But the daylight still shortens, the time change still lands, and about a third fewer daylight hours in December than in June is a real physiological input regardless of how mild the afternoons are.
The people who tend to get caught hardest are the ones who moved here from somewhere darker and concluded the problem was geographic. When the pattern shows up anyway, in a place with palm trees, it is disorienting in a way that makes people slower to name it and slower to ask for help.
What to actually do in September
Daylight saving time ends on November 1, 2026, which is about seven weeks out. That is enough runway to matter.
- Name last year’s pattern. When did it start, what went first, sleep or motivation or appetite, and what was already gone by the time you noticed?
- Build the routine now, while it costs less. Morning light exposure, movement, and a stable wake time are far easier to establish in September than to invent in December.
- Protect the social calendar before it thins. Withdrawal is the engine. Standing commitments that are hard to cancel do more than intentions.
- Handle the medication conversation early if medication has been part of what worked before. Starting that in September, coordinated with psychiatry, is a different experience from starting it in the middle of a bad December.
- Get the beliefs on the table with a therapist while you can still examine them from outside, rather than while you are living inside them.
When to bring in help
If this is a repeating annual pattern, that alone is enough reason. A predictable recurrence is precisely the case where starting before onset makes the most difference, and it is the case where waiting costs the most.
Come sooner if your sleep or appetite has already shifted, if you have started declining things you would normally say yes to, or if last year was bad enough that you are dreading this one.
Our seasonal affective disorder care uses CBT adapted for seasonal patterns, alongside routine and light strategies, coordinated with psychiatry when medication is part of the picture. It sits within our broader depression treatment, in person in the South Bay and online across California.
If you are having thoughts of harming yourself, call or text 988 to reach the Suicide and Crisis Lifeline, or go to your nearest emergency room.