Sleep vs food...

Im curious if anyone knows the answer. I know we need to feed them every 3 hours, but they also need sleep too. So my dilemma is, if they take the three or close to the three hours after feeding to go to sleep do I wake them after they barely slept to eat again? Or do I let them sleep? Sleeping helps growing too... They are also preemies born at 31 weeks are now a week before their due date, so they have lots of catching up to do... Anyone know the answer? Tia!