During non-REM sleep, visual areas of the brain exhibit an excitation/inhibition balance indicative of increased plasticity. REM sleep appears to be essential for people to reap the benefits of the increased plasticity that occurs during NREM sleep. Source: “20 Hz Steady-State Response in Somatosensory Cortex During Induction of Tactile Perceptual Learning Through LTP-Like Sensory Stimulation”. by Hubert Dinse et al. Frontiers in Human Neuroscience.