I know this was originally about forced landings at night, but enough people have broadened it out to general risk assessment of night flying. So I'd like to point out that the chances of an mid-air-collision seem to be much lower at night:
1. There are fewer people up (the sky is even bigger)
2. They'll (99.9999% of the time) have their lights on, so you'll see 'em sooner.
For me this kinda balances the FL problem, so (when I get the qualification), I'll do Xcountries at night. I will however extend the passenger brief to attempt to inform them better.
Steve R