nursing has it's own built-in soporific: Try feeding "oxytocin sleep" and "prolactin sleep" into PubMed - there seem to be some indications there of a link between prolactin (stimulates milk production) and oxytocin (stimulates milk release) and sleep. Then again, oxytocin gets up to all sorts of stuff and is regulated by all sorts of stuff, so whether this is an adaptation or a coincidence is not clear.

I was amused by an interesting paper title I found on one of those searches: "Oxytocin: an extremely potent inducer of penile erection and yawning in male rats."