Is it easier to be depressed at night? Is it harder to be optimistic when surrounded by darkness? Or is it because all the distractions of the day can no longer be used as a buffer?
Well there's something called seasonal affective disorder, where people get depressed without sunlight. People can feel instantly energized when exposed to lots of light (sunbox). It seems reasonable that if you can feel instantly better, you can feel instantly worse.
I haven't noticed gratter depression in the dark, most of the time. The effect of winter does seem to imply that darkness is a little bit more depressing for me. I really love walking around at night though, or I did when I lived in a small town anyway.
Around here, people get shot at night, so no more walking for me.
I'm reading all of this and wondering if some of the problem isn't social conditioning (or possibly natural). After all it is always said as "what shall I do today?" with a link to doing things whilst the sun is up. Ergo at night you can do nothing about what troubles you... perhaps indicating that those who find the night depressing should spend more of the day sorting stuff out so they can enjoy the evenings?
Anyhow personally I found that I'm a night owl overall with my favourite time being in and around the witching hour. Oddly I also enjoyed midday in Vegas though.... I shall stick to my favourite answer and just climb back on this fence.