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. The baby moves quite a bit, may grunt and open eyes briefly, but may go back into light sleep several times before really waking up. Young preemies may be in this state quite a bit. A young preemie may spend only 2-5 minutes in deep sleep before going back into light sleep.