SAD is, as you know, more generally associated with Fall and Winter. Some people prefer darker nights because they feel more 'awake' at night-- they are night owls, and prefer to the darkness.
For some people this has to do with their sensitivity to light, and for some it's their circadian rhythm.
Some of those things have loosely been associate with modd disorders (depression and bipolar -- even at very low levels) because mood is very much effected by light/dark as we see with SAD.
I hope this answers helps! Email back with any follow up questions :-)