For the same reasons they sleep better next to an adult body than alone: biologically, it's exactly what we need to do.
2000 years ago, the hyenas could come and take a sleeping, unattended baby away. Both moms and babies know this, instinctively.
Today, newborns still lack the ability to regulate their own body temperature and breathing appropriately so they need their mother's bodies to help them, and moms know this instinctively. They know their baby is safest when they're holding him. Just as he knows he's safest when being held.
Be sure you're sleeping in a safe bed, make sure you're only sleeping (not unconscious from alcohol, cold meds or sleeping drugs) and make sure everyone else you share sleep with is in the same condition. Then, relax knowing that when you're sleeping, your're asleep -- not unconscious. For the same reason you don't fall out of bed, you're alert all the time to your baby's wellbeing.
at 12:05 PM on Jun. 8, 2009