I used to work in the transportation industry, and found out thru the Safety end of it about the Circadian Cycle. That is the natural rhythm governed by light every living thing has in relation to sleep/wake. It seems to be genetic, and could explain why some people require less or more sleep. With out enough sleep, a person is as unalert and accident prone as some one who has been drinking.
For me, personally, lack of light can be an issue. I have a 500w HID system, that I garden with. (Hanging with my plants helps, too.) I find that the lamp can help reset my clock, improve my mood, and if I get too much light, I get really amped.