I’ve read a magazine (I forget the name of the magazine; it is about ‘mommy & kids’ magazine). It is said that during sleeping time, the baby's brain cells are developed and grown, especially at the golden years (1 – 3 years old). So that is why, deep sleep is good for baby’s brain growth.
However, there is one thing as a big problem for mommies (especially me, actually), because my baby already learn to play, by himself or with his friends, sometimes he refuse to go to sleep. He wants to play :). That is why; I have to find some ways to make my baby sleep with enough time and sleep well at night.