The reticular arousal threshold during the transition from slow wave sleep to paradoxical sleep in the rat.

The short-lasting intermediate stage of sleep occurring between slow wave sleep and paradoxical sleep showed frontal cortex spindles of higher amplitude and longer duration than during slow wave sleep, and hippocampal theta rhythm of lower frequency than during paradoxical sleep. During the intermediate stage, the mesencephalic reticular arousal threshold was slightly higher than during the slow wave sleep and much lower than in the paradoxical sleep. Thus, following this central reactivity test, the intermediate stage seems to be functionally nearer the slow wave sleep than the paradoxical sleep.