American Academy of Pediatrics (AAP)
All humans, adults and children, go through sleep cycles that include arousals and wakings. For that reason, the AAP's Guide to Your Child's Sleep encourages parents to help infants over three months old learn how to soothe themselves so that they can quickly fall back asleep between one of those cycles.
The key once again is to have a routine for naps and bedtimes. Starting when the baby is about six weeks old, you should go through your pre-bed routine of singing song(s), reading a book, rocking, etc., but keep the baby awake until he is in the crib. The nursery should be dim and quiet. Offer him a “lovey” if it helps.
If the baby wakes up in the middle of the night, don't rush into the room as soon as he cries, as he may fall back asleep on his own. If he doesn't, go in to comfort him, but don't pick him up. Pat him gently, speak in a low voice, and leave once he calms down.