My problem is the opposite. I can fall asleep very easily. Over 8 or 9 o'clock in the evening, if I'm doing something that doesn't require too much attention (like watching a film or something like that) I start to feel asleep and it's very hard not to.
The funny thing is that, if I let myself sleep for a few minutes (5-15), then I wake up fresh and ready to be awake a bunch of hours more (and it doesn't prevent me from getting asleep later).