Seasonal affective disorder (SAD) is a temporary state of depression that sets in during a certain season, typically winter, and then remits during the rest of the year. The disorder is most likely to occur in young adults and women.

For most people with SAD, onset occurs during the winter months as the days become shorter. The lack of sunlight during this time may affect our circadian rhythm (sleep-wake cycle), leaving us feeling groggy and sleepy at inappropriate times. The shorter days/longer nights may also lower our body’s production of serotonin, the feel-good neurotransmitter, while raising our production of melatonin, the sleeping hormone. Another contributing factor may be that the lack of adequate sunlight lowers our levels of vitamin D, a nutrient known to play an important role in staving off depression.

In the U.S., prevalence of classic (winter-based) SAD increases with latitude. For example, about 1.4 percent of people living in Florida suffer from SAD compared to 9.7 percent of the population in New Hampshire. More rarely — in about 10 percent of SAD sufferers — depression sets in during the summer months. Some studies have found that summer SAD is more common in populations closer to the equator. Experts are unsure of the cause of summer SAD, but increasing heat and humidity may play a role.

Symptoms of SAD are similar to those of clinical depression. These may include a lack of energy and pleasure, feelings of hopelessness, oversleeping, difficulty concentrating or completing tasks, cravings for carbohydrates and withdrawal from social activities.

Treatment for classic SAD often involves bright light therapy, which appears to correct a lagging circadian rhythm. Other types of treatment may include medication, cognitive-behavioral therapy (CBT), vitamin D supplementation, exercise, carefully timed melatonin supplementation or ionized air treatments.

Example: Janice often dreaded the holidays as her symptoms of seasonal affective disorder always set in by the end of December.