There's some science, too (Destiny)

by marmot 1333 , Tuesday, December 01, 2015, 21:03 (3069 days ago) @ TheeChaos

Infants under 6 months of age have 'active sleep' which is basically REM sleep (dream sleep) but they don't have the same paralyzing chemicals to stop their muscle movements that older humans do. That's why young babies throw their arms and kick their feet while sleeping.

For example, when your baby has a dream (s)he is punching Xur because he tricked them into spending all their strange coins on Y1 exotics, (s)he will actually flail her arms. You, being the mature adult you are, will only dream this movement and your fists of fury won't lash out, because the only muscles active during REM sleep are your eye muscles, your diaphragm, and your heart.

Infants also sleep and wake a lot more per day (I'm sure you're saying DUH loudly right now,) but there has been interesting research into this that results in confusing graphs of wakefulness.