The Two Sleep States

We named this strange state "paradoxical sleep ". It is also called deep sleep, fast-wave sleep, rapid-eye-movement (REM) sleep and dreaming sleep, whereas the lighter sleep that precedes, it is often called slow-wave sleep. We consider paradoxical sleep a qualitatively distinct state, not simply a deepened version of the first stage of sleep. Very schematically (for the cat) we can describe the three states-wakefulness, light sleep and paradoxical sleep in the following physiological terms. Wakefulness is accompanied by fast, low-voltage electrical activity in the cortex and the subcortical structures of the brain and by a significant amount of tonus in the muscular system. The first stage of sleep, or light sleep, is characterized by a slackening of electrical activity in the cortex and subcortical structures, by the occurrence of "spindles," or groups of sharp jumps, in the brain waves and by retention of the muscular tension. Paradoxical sleep presents a more complex picture that we must consider in some detail.

We can classify the phenomena in paradoxical sleep under two heads : tonic (those having to do with continuous phenomena) and phasic (those of a periodic character). The principal tonic phenomena observed in the cat are fast electrical waves (almost like those of wakefulness) in the cortex and subcortical structures, very regular "theta" waves at the level of the hippocampus (a structure running from the front to the rear of the brain) and total disappearance of electrical activity in the muscles of the neck. The principal phasic phenomena are high voltage spikes, isolated or grouped in volleys, that appear at the level of the pons and the rear part of the cortex (which is associated with the visual system). These spikes make their appearance about a minute before the tonic phenomena. Just as the latter show up, the peripheral phasic phenomena come into evidence : rapid eye movements, clawing movements of the paws and soon. The high-voltage spikes during paradoxical sleep in the cat come at a remarkably constant rate: about 60 to 70 per minute.

Our continuous recordings around the clock in a soundproofed cage have shown that cats spend about 35 percent of the time (in the 24-hour day) in the state of wakefulness, 50 percent in light sleep and 15 percent in paradoxical sleep. In most cases the three states follow a regular cycle from wakefulness to light sleep to paradoxical sleep to wakefulness again. An adult cat never goes directly from wakefulness into paradoxical sleep Thus we find that the two states of sleep have well-defined and clearly distinct electrical signatures. Equipped with this information, we are better prepared to search for the nervous structures and mechanisms that are responsible for sleep and dreaming.

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