Normal SAD is a mental disorder where the sufferer experiances extreme depression and mood swings during the darker, colder winter months.
Reverse SAD is a much rarer form, where the sufferer experiances extreme drepression and mood swings during the hotter summer months.
People may equate the summer months with swarms of insects, heat rash and sunburn and get depressed that it may happen when ever summer comes around. Overweight people or people with heat sensative skin (excema for example) may dislike the heat too.
It is unknown why people suffer from SAD at all. It could be a mental difficency, a lack of vitamin D (vitamins we get from the sun), or a programmed state of mind.