Non-rapid eye movement sleep activation occurs during NREM that is comparable to the type that occurs during REM. It is this sub-cortical activation that results in dreaming during the NREM stage during the morning hours. During non-REM sleep, the tonic drive to most respiratory muscles of the upper airway is inhibited. This has two consequences: However, because the diaphragm is largely driven by the autonomous system, it is relatively spared of non-REM inhibition. As such, the suction pressures it generates stay the same. This narrows the upper airway during sleep, increasing resistance and making airflow through the upper airway turbulent and noisy. For example,
Rapid eye movement sleep reduced need for wakeful food foraging. Other theories are that REM sleep warms the brain, stimulates and stabilizes the neural circuits that have not been activated during waking, or creates internal stimulation to aid development of the CNS; while some argue that REM lacks any purpose, and simply results from random brain activation. Recognition of different types of sleep can be seen in the literature of ancient India and Rome. Observers have long noticed that sleeping dogs twitch and move but only at certain times. The German scientist Richard Klaue in 1937 first discovered a period of fast electrical activity