sweetpea I keep my girls on a strict routine. I have to or I will be feeding 24 hours a day if I let them demand feed. With twins (and I don't imagine a singleton is different), the advice is to keep them topped up with milk so they sleep better at night. The twin parents I have spoken to (one of them is my GP) all say to keep them regularly fed/topped up in the day, even if that means waking them. I feed the girls at 7:30am, 10:30, 2pm, 5pm (this is a split feed, 30z at 5pm & 2oz at 6pm), then they wake around 10:30pm. I let them wake themselves at night. Currently it is around 4:30am. I always wake the other twin (or I would be up and down like a yoyo).
Before I took this advice and let them demand feed (like the NICU suggested), I never got any sleep. At one point, I did 6 night feeds between 11pm & 5am (3 per baby). I was exhausted and broken.
With regards to them snacking. Like horsey said, feed them when they are awake. When I wake them, I open the blinds & curtains, open their sleeping bags, sing to them and only take them out their cots and feed them when they are completely awake. If he drifts off to sleep, open the legs of his babygro, wind him, change feeding positions. Keep him awake for him to take his feed.
Hope this helps :)