Question

The human infant spends about half of its sleep time in rem. What is the understanding of why this is the case?

Answer 1

This occurs because REM sleep may provide at least part of the stimulation necessary to correctly develope the immature brain of infants.

REM sleep provides the electrical activity necessary to establish neural connections that include the  development of synapses in the brain. These neurological process enables development of sensory, motor, learning and memory system.