I think you have to be tired enough to fall asleep. If you're hopped up on sugar and caffieen, then you're not going to sleep. But if you calm yourself down enough and you're comfy, then you could fall asleep even if you're not tired (peaceful and comfy will cause you to become tired and fall asleep I'd assume)

As for light... if you can fall asleep in it normally, then it shouldn't be a problem