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.