Is it possible to sleep train (using any method) to get LO's to sleep longer stretches? If they have no problem falling asleep on their own to begin with, but wake up too early - not really hungry and no real needs to be met.

When you know it's too soon by looking at the clock, do you let them CIO? Would that ever work to 'teach' them to stay asleep longer or does CIO only work for getting them to fall asleep on their own?