Manifestations of "activated" sleep in the rat.

A sleep phase similar to feline "activated" sleep was found to occur regularly in normally sleeping rats. However, at these, times the electroencephalogram differed from the waking or "activated" pattern in being dominated by 6-to 8-per-second waves. Instead of assuming a very relaxed posture, as cats in "activated" sleep do, the rats tended to undergo shifts in muscle tone, occasionally of considerable magnitude.