A baby's sleep is not affected by what it eats, but rather determined by it. As adults we know the difficulty that the wrong meal or no meal can make to getting a good night's rest but unfortunately it's tough to explain this to a baby who is far from rationale, but rather driven by biological needs. A baby's main source of nutrition for the first two years of life is milk.
A newborn baby has a stomach the size of a grape. In the first few days, a baby does not drink more than a few milliliters for the stomach to gradually stretch. By two weeks old, a baby's stomach is the size of an egg and can handle a good breastfeed (60 - 100ml). As a baby's stomach stretches it is able to take in a higher volume of milk and therefore stay fuller for longer. A full baby is a content baby and this is why babies will slowly start to stretch their sleep too.
Overeating creeps in when mothers try to get their babies to finish an unrealistic amount of breast milk or formula at each feed. The temptation is to feed them more so they sleep more but the consequence is often discomfort, reflux and vomiting. Reflux is when the milk cannot be contained in the stomach and moves up into the oesophegus. This can be extremely painful as the acid from the stomach often comes along for the ride causing severe heartburn. Vomiting occurs when the reflux flows all the way up into the nose and mouth.
A large vomit may mean that the bulk of the meal is lost and that the baby will actually not retain the necessary amount of milk to feel full. This results in a vicious cycle of feeding and vomiting.
Alternatively, a baby may overfeed due to pain. Babies cannot distinguish pain from hunger and so their response to both stimuli is to soothe themselves the only way they know how - to suck. A baby who is experiencing reflux may suck in reponse to the pain from the heartburn causing only more discomfort as the stomach cannot hold the milk and it can be forced into the gut. When undigested milk enters the gut it causes cramping which is often called colic.
If your baby is sleeping poorly (see blog on sleeping norms for all ages) and you suspect that your baby is in pain or experiencing reflux, vomiting or colic then visit your local clinic nursing sister or paediatrician who will be able to investigate further.