SOME MODERN THEORIES OF SLEEP
I have an exposition of sleep come upon me.
SHAKESPEARE.
There have been almost as many theories of sleep and its causes as
there have been investigators, but these theories may be grouped under
a few main heads:
Physiological, or that which has to do with some bodily conditions
only, and which made men think that sleep was dependent upon the
circulation of the blood or upon decreased consumption of oxygen, was
an early one of these theories. It has had many advocates and has led
to many interesting experiments that have increased the sum of general
knowledge, although they have not explained sleep.
Delicate instruments, with formidable names, have been invented
and successfully used to measure the intensity of sleep and to
note its phenomena. Two of the experimenters—C. E. Brush, Jr., and
R. Fayerweather of the Physiological Laboratory of Johns Hopkins
University—through long, intricate and exhaustive experiments, have
found that sleep is most intense and the pressure of blood in the
arteries lowest during the first half of the sleeping period. After we
have completed the first half of our sleep, the intensity or soundness
of sleep becomes less, and the pressure of blood in the arteries
continues to increase up to the moment of awaking.
It is interesting to learn that the moment when we are most soundly
sleeping is at the end of the first hour of sleep, and that the
blood-pressure has at that time reached its very lowest point. Messrs.
Brush and Fayerweather report that, during the first few hours of
sleep, the blood-pressure continues to fall and then begins a gradual
rise. The tendency is to more and more rapid flow of the blood, but
this rise is not steady or regular, because it is broken by long waves
when the force of the circulation falls and the pulse is weaker than it
was a moment or two before. The rapidity of the blood-flow is greater
on the moment of awakening than just before dropping to sleep. This
increase is not sudden, but is the culmination of the rise that begins
a few hours after we fall asleep. (See Appendix B.)
The intensity or depth of sleep is shown by a curve that looks like a
pile of sand with the top scooped off. It increases rather slowly, in
most cases, for the first quarter-hour: then quickly, so that half an
hour later the person is most “sound asleep.” He stays so, on the
level top, for about half an hour. That is the time that wise burglars
and late husbands choose to steal into the house, about an hour after
everyone is asleep. After that time the sleeper reverses the process of
falling into deep sleep by getting nearer to waking for half an hour
and then getting, at first rapidly, nearer to waking for two or three
hours. In the last three or four hours healthy and normal persons reach
about the same proportions of time and intensity of sleep, so that the
Indian-bow-shaped curve fairly represents how long it takes everybody
to deepen his sleep. Kohlschütter found how great an intensity of sound
was needed to awaken a sleeper at different periods throughout the
night. His curve thus made tallies very exactly with that of Brush and
Fayerweather, obtained in quite a different way.
Some other investigators have pointed out that, interesting as
this theory is, it proves one thing about as completely as it does
the other. For, while it is plain that sleep and the great fall in
blood-pressure exist at the same moment, it is not conclusively shown
which is cause and which is effect. Does sleep cause the fall in
blood-pressure, or does the fall in blood-pressure cause sleep? The two
are coexistent, but who can say which begins first?
It looks as if sleep might be more justly considered the cause, if
one takes the sleeping-position, and maintains the attitude of mind
suitable to induce sleep, blood-pressure grows less, even though the
patient does not actually fall asleep.
Under this physiological view must come also the chemical theory based
on the fact that we consume more oxygen during the day, thus forming
carbon dioxide and other poisons which cause sleepiness. During the
night we absorb oxygen, building up the tissues, and eliminating the
poisons of the waking hours.
The poisons which are the result of the consumption of oxygen cause
fatigue, and according to Preyer, a European authority, “sleep is the
direct consequence of fatigue, or rather of the fatigue products in
the blood.” His contention is that, if lactic acid and other chemical
products of the consumption of oxygen in the body were injected
artificially, sleep would follow. Experiments in this direction made by
Preyer, Fisher, and L. Meyer have yielded such contradictory results
that the theory is not proved thereby.
The idea that sleep is the result of poisons in the system takes us
into the pathological theory of sleep, which regards it as a sort of
disease like epilepsy or auto-intoxication. We produce by our own
activities the poisons which cause insensibility until the system
cleanses itself. Professor Leo Errera of Brussels says that “work in
the organism is closely bound up with a chemical breaking down.” Among
the products of this breakdown are “leucomaines,” the scientific
name for poisons formed in living tissue, and just the opposite to
“ptomaines,” which, however, are also virulent poisons.
Professor Errera tells us that, during our waking hours, we produce
more leucomaines than the oxygen we absorb can destroy. This excess is
carried along by the blood and held by the brain centers, and in time
produces sleep, just as any poisonous anæsthetic, such as morphine,
would produce sleep.
While we are sleeping we absorb much oxygen and we recover from the
effects of our self-intoxication. Errera maintains that work, fatigue,
sleep, and repair are not merely successive events, but phenomena
chained together in a regular and necessary cycle. He explains
sleeplessness due to overfatigue on the theory that small doses of
poisons induce sleep and large doses induce excitement and even
convulsions.
Manacéïne points out that this theory is good from a purely physical
standpoint, but does not explain our power to postpone sleep or the
faculty of waking at a fixed hour. We can do both, and any adequate
theory of sleep must explain why we can control the tendency to sleep,
but cannot control the symptoms of ordinary poisoning.