Lack of light and seasonal depression—what’s the link?

People troubled by depression usually experience their dark moods in an on-again, off-again fashion. In that respect, seasonal affective disorder (SAD) differs only in that the oscillations follow a seasonal schedule, with the depression usually starting in the fall and lasting through the spring. Lack of light is often blamed for SAD, but just how darker days cause depression in SAD sufferers is still in question, reports the January 2008 issue of the Harvard Health Letter.

Experts debate whether it has been proved that lack of sunlight in winter triggers SAD, but there's certainly circumstantial evidence to support the connection. How might lack of light cause depression? The Harvard Health Letter discusses three theories: