changed sleeping pattern at thirteen weeks?

my baby boy is thirteen weeks old and he has been only waking once a night about four-five o clock. he is breast fed but has one bottle at about ten. he is a big boy at fourteen pounds and has recently started to wake up at half one and five, i dont know why he has started waking the first time cos nothing has changed.

just wondering if anyone has experienced the same and can suggest ways to get it back to once a night or even all the way through because i know a lot of babys are at this age!thankyou, Lauren:\?