3 weeks and won't wake for feed at night

This is my first baby and he's now 3 weeks old but since be been on 3 floz he doesn't seem to wake up for feeds at night , he wakes up during the day for feeds but don't understand why he doesn't at night I have to wake him up and even then he doesn't feed proply because he's half asleep. When I was at the hospital the midwifes say they can't go more then 4 hours with out feeding but I'm worried because if I let my boy he would happily sleep for about 6-7 hours , any advice ?