I've taken the Crescent numerous times from DC to Greensboro, and I second dmitzel's advice. The dining car will be closed as you leave DC but will reopen about 30 minutes after departure. I recommend the desserts.
For a midnight snack, you might want to bring your own food; even though the lounge car will be open until 11 pm or so, the food is pricey and selection may be limited. Since you're in a sleeper, you can even BYOB (I believe).
Finally, your sleeper tickets give you access to the Club Acela in Union Station. I haven't been, but it should be worth a visit.
Enjoy your trip!