A reader asks:

I get the impression from everything I have read that exposure to light at the time of waking is important for fighting Seasonal Affective Disorder (SAD). However, in the winter, when I am suffering, I tend to wake up after sunrise to get exposure early in the day.  On the other hand, in the summer I often wake up well before sunrise by a couple of hours and sit in a room that is dark except for the light coming from my computer screen. In summer I feel much better. Why is that?

Answer:

Nice question! As summer sunrise moves earlier, it can cause the internal circadian clock to shift earlier by varying amounts in different people. If you’re getting a big clock shift, it can cause you to awaken substantially earlier than sunrise itself. Such a shift is not necessary for “feeling much better.” However, we think the very process of shifting earlier, even by smaller amounts, has a therapeutic effect. Even with artificial light therapy in winter, some people find themselves waking up in the dark — well before the light therapy session has been scheduled. Although they may feel better while this is happening, it is unnecessary. By reducing the dose of the light setting (for a shorter period, at lower intensity, or with a slightly later wake-up time), they can find a combination that allows them to sleep longer, getting up for the light.