Can you please give me a technical description of sad (seasonal affective disorder)?

It's gone!: Years ago it was a separate diagnosis, but in the recent diagnostic manual it is no longer classified as a unique mood disorder, but is a specifier for recurrent major depressive disorder called "with seasonal pattern" that occurs at a specific time of the year (usually winter) and fully remits otherwise. It seems to be caused by the lesser amounts of sunlight and the affects that has on the body.

SAD: SAD symptoms usually occur with change in season (often fall and winter) & can include carbohydrate craving, increased appetite with weight gain, low energy level, irritability, moodiness, sleeping more, heavy feeling in extremities, being overly sensitive to rejection and it can impact relationships.