At first the states are hard to tell apart, but they become more clear as the baby grows. The amount of time a baby spends in each state also changes as the baby grows. Both of these patterns of change reflect the gradual maturing of the brain and nervous system. The baby is very still. Every now and then there is a sigh or startle.
Breathing is quite even.
Deep sleep is thought to be the more restful stage of sleep, and important for growth. Young preemies have very little deep sleep, and there may be more little movements and less even breathing. Young preemies spend most of their time in light sleep. Very young preemies don't move as much or have as much REM as older preemies. It is hard to tell whether the baby is asleep or awake, as when the baby is beginning to wake up.