Your previous report said that exposure to morning sunlight can help fight depression. Does midday or evening sunlight help, too? —J.M., via e-mail

Yes, but morning exposure appears to work best. In theory, the brain may be more sensitive to morning sunshine because it uses the first light of day to keep track of time. Lack of sunlight in winter disrupts the brain’s internal clock and delays the brain’s release of numerous mood-influencing substances, researchers believe. That can lead to daytime sleepiness, and in susceptible individuals, either seasonal affective disorder (SAD, or winter blues) or typical depression.

Exposing the eyes to sunlight or a full-spectrum electric light box for as little as 30 minutes in the morning helps reset the internal clock and restore normal moods. Exposure later in the day isn’t as helpful, possibly because it doesn’t advance the brain’s clock. A small percentage of SAD sufferers may benefit more from evening-light exposure because their biological clocks run fast, not slowly, in the winter.