Just like grown-ups and older kids, infants switch between REM (light) sleep and non-REM (very deep) sleep several times during the night and during naps.
The big difference between baby sleep and adult sleep is, until they’re around two years old, babies log more REM sleep, which is when they dream, and also when they may wake up a bit. It’s during such partial arousals they may realize they can’t get back to sleep–unless, that is, they’ve learned how. That’s why it’s important for a baby to be able to put herself to sleep–otherwise she’ll need you to come help her every time she wakes up.
Excerpt from “52 Sleep Secrets for Babies“