My baby is 3.5 months old and around 17lbs. He wakes the same sort of times. He is a big boy and I am just going with it. My other 3 children didn't sleep through the night properly until they were 2. Easy to say, but the books are generalising. All babies are different as are adults. I think sometimes we stress ourselves out more by worrying our baby isn't fitting into the pattern rather than trying to go with the flow. I keep remembering what my sister said that a baby's tummy is the size of their clenched fist. Try to get to bed as soon as the baby goes down or at least rest.