During your leave, your baby had access to you 24x7, but now you are gone for a large part of the day. Babies seem to have a “mommy quota”, and if they don’t fill it during the day, they will seek it at night. It’s not about the milk; it’s about being with mom.
This isn’t meant to induce guilt – it’s meant to help you understand a change in behavior. What often happens at this point is if baby has been sleeping in his own crib and/or room, he moves to co-sleeping with mom and dad at night. This helps minimize the impact of more frequent waking, and everyone gets to make up for closeness that didn’t happen during the day.
It turns out that a lot of dads embrace the co-sleeping because they tend to spend more time away from the baby anyway. Note: This may be the time to invest in a king-sized bed, or to move a couple of mattresses onto the floor. If you start out with the bed on a frame, consider moving the mattress to the floor when your baby starts rolling over so you don’t spend the entire night worried about him falling out of the bed. On this last point, I am speaking from personal experience!