There’s no magic about the date of time change. Some people with SAD, or those with the less severe winter doldrums, begin to feel down well before the time change, while others feel well till much later in the season.
There are two strategies to consider: (1) On the basis of your personal seasonal history, start the treatment shortly before you usually begin to feel fall/winter symptoms — say, one week beforehand. This is a “prophylactic” strategy aimed at preventing the onset of the winter episode altogether. The drawback is that you cannot predict exactly when the episode would begin, so you might be engaging in unneeded treatment.
(2) Wait till mild symptoms begin to set in, and then immediately start the treatment. Most often, people begin to feel sluggishness, increased sleep pressure and heightened appetite for carbs weeks or even months before depressed mood appears. If you use this strategy, you can be more confident that you are treating actual symptoms of the fall/winter episode and you can verify subjective improvement once the treatment starts. Usually, we recommend the latter strategy.