If you feel that winter makes you sad and depressed, it’s probably not just in your head. There is a reason behind this and it is called Seasonal Affective Disorder (SAD). This condition, which is more common in women than in men, is the reason behind the sudden change in your behaviour during winters and summers. Notably, SAD is not affected by dates but the temperature and thus, should not be confused with "holiday blues", which are the feelings of sadness or anxiety brought on by stresses at certain times of the year. According to National Institutes of Health (NIH), SAD starts in late fall or early winters and goes away during spring or summer. People suffering from depression or bipolar disorder, especially bipolar disorder II, are also more susceptible to this disorder. What Are Symptoms Of SAD?Persistent sad, anxious, or “empty” mood most of the day, nearly every day, for at least 2 weeksFeelings of hopelessness or pessimismFeelings of irritability, frustration, or restlessnessFeelings of guilt, worthlessness, or helplessnessLoss of interest or pleasure in hobbies and activitiesDecreased energy, fatigue, or feeling slowed downDifficulty concentrating, remembering, or making decisionsChanges in sleep or appetite or unplanned weight changesPhysical aches or pains, headaches, cramps, or digestive problems that do not have a clear physical cause and do not go away with treatmentIn extreme cases, it could also lead to death or suicide or suicide attemtps. Science Behind SAD While there are various studies that reveal details about winter sadness, not much is explored about the less-common version of this disease, known as Summer-pattern SAD. Research has shown that people with winter SAD have lesser levels of Serotonin in their heads. Serotonin is responsible for regulating moods. It is a well known fact that sunlight affects the level of this hormone in the body. Therefore, having shorter days and fewer sunlight hours interferes with the production of serotonin, leading to low mood. Moreover, Vitamin D deficiency may exacerbate these diseases. In addition to vitamin D consumed in food, the body produces vitamin D when exposed to sunlight on the skin. With less daylight in the winter, people with SAD may have lower vitamin D levels, further reducing serotonin activity.Another hormone that plays an important role here is Melatonin. Several studies suggest that both forms of SAD (winter and summer) relate to altered levels of melatonin—a hormone important for maintaining the normal sleep−wake cycle. People with winter-pattern SAD produce too much melatonin, which can increase sleepiness and lead to oversleeping.