Ah, good topic, if I say so myself - this is something that affects me a lot
In the morning the first thing I usually do is open the blinds after I get dressed. I like the sun coming in anyway, but especially when I'm feeling depressed; if I don't have the sunlight coming in/my room is dark, the depression feels like it will suffocate me. Weird, no? It's such a strong feeling that I actually usually lift my blinds up completely to allow the maximum amount of sunlight to come in. My mother is the opposite, she likes the dark and keeps them closed whenever she can get away with it - ugh! The horror.
I have to have mine shut at night too. Any little light bothers me at night. Has to be completely dark. I keep something over my alarm clock face to block the glow - those things are way too bright, even the ones with dimmers. I've read that even tiny sources of light output in bedrooms can disrupt the production of melatonin in the brain at night, preventing or making falling asleep difficult.