Naps make time pass by much faster than staying awake, which means they really are a much-needed thing when flying. Some people are quick to nap as soon as they land in their seats, while others take some time to get settled before bedtime. Waiting is actually a better idea because falling asleep quickly won't give you the time to regulate your body during takeoff. Also, you might end up missing the snack service.
Falling asleep before takeoff also won't give you a chance to pop your ears when the cabin air pressure changes, and that could leave you with a nasty headache after waking up.