​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 depression usually starting in autumn and lasting through to spring. Lack of light is often blamed for SAD. Experts are still debating whether the lack of sunlight in winter triggers SAD. Light therapy, which involves sitting in front of a bright light for a short time each day, helps some people who suffer from SAD. But anti-depressant medications may work just as well.