Seasonal Affective Disorder (S.A.D.) is a depressive disorder that affects some people during certain seasons of the year, most typically winter and is sometimes called the winter blues.
Though the diagnosis of such a disorder may seem trivial to some, there are many scientific theories as to why the disorder exists, including: less sunlight (intensity), deficient vitamin D, a leftover evolutionary trigger in the human genome related to hibernation, and difficulty in adjusting to the shift of daylight and nighttime hours.
There are also a number of side-effects that are reported with the disorder which include: a tendency to oversleep, restricting social interaction with family and friends, loss of energy, reduced sexual drive, and sluggish cognitive process.
There a number of treatments for the disorder, which is typified by full remission of systems once the given season is over, such as lux-lamps (which provide an artificial source of ultra-violet light), ionizers, and various therapeutic treatments. However, many idividudals affected by S.A.D. are also affected by bipolar disorder, and major depressive disorder, which can increase feelings of isolation, nihilism, reduce hopefulness, and generally compound pre-existing depression.