(HealthDayNews) -- If your mood seems to drop with the amount of daylight, you may have a condition called seasonal affective disorder, according to the National Mental Health Association.

As the seasons change and sunlight decreases, your "biological clock" can drift out of sync with your daily schedules.

The most difficult months for most SAD sufferers are January and February, and younger people and women are at greater risk.

Phototherapy, also known as bright light therapy, has been shown to suppress the brain's secretion of melatonin -- a hormone believed to play a part in causing the condition.

Many sufferers have benefited from use of a simple light box device -- containing a bank of white fluorescent lights on a metal reflector. For mild symptoms, spending time outdoors or in rooms with bright sunlight also may help.