Macbeth does murder sleep,’ the innocent sleep;
Sleep, that knits up the ravell’d sleave of care,
The death of each day’s life, sore labour’s bath,
Balm of hurt minds, great Nature’s second course,
Chief nourisher in life’s feast.”
And again, when the strenuous life of the great Bolingbroke has at last
overtaxed his brain, and he can no more find rest and unconsciousness
at night, Shakespeare makes him say—
“How many thousand of my poorest subjects
Are at this hour asleep! O sleep, O gentle sleep,
Nature’s soft nurse, how have I frighted thee,
That thou no more wilt weigh my eyelids down,
And steep my senses in forgetfulness?
Why rather, sleep, liest thou in smoky cribs,
Upon uneasy pallets stretching thee,
And hush’d with buzzing night-flies to thy slumber,
Than in the perfum’d chambers of the great,
Under the canopies of costly state,
And lull’d with sound of sweetest melody?”
Poets have as a rule been too ready to make much of the likeness of
sleep and death, whereas there is an absolute difference in their mere
appearance. Sleep makes even those who are ill-favoured and coarse look
beautiful, imparts to its subjects a graciousness of expression and of
colour, and a gentle rhythmic movement, whilst suffusing them as it were
with an “aura” of contented trustfulness. These things are far from the
cold stillness of pallid death. And this depends upon the fact that in
sleep, though many of the activities of the body and mind are checked,
and even arrested, there are yet still present the never-ceasing pulse
of the heart, the flow of the blood, the intake and output of the
breath, and a certain subdued but still active tension of muscles, so
that though the body and limbs are relaxed they never assume the aspect
of complete mechanical collapse which we see in death. The pupils of the
eyes are strongly contracted during sleep, not relaxed and expanded as
are those of wide-awake people in the dark. There are some well-known
works of art—both painting and sculpture—in which the dead are not
truly represented, but are made to retain the resistfulness and pose of
living men and women; others show true observation in presenting the
startling and distinctive flaccidity of the newly dead, which is
followed after a few hours by the equally characteristic rigor mortis,
or stiffness of the dead. There are many fine studies of sleep by
sculptors, but none which to my thinking so delicately and truthfully
present its most beautiful and peculiar effects on the muscular “tone”
as a work in the Luxembourg Gallery in Paris, called “Le Nid”—a baby of
a year old and a little girl of three or four years, asleep side by side
on the cushion of a capacious arm-chair. The pose and the details of
muscular relaxation differ greatly and characteristically in the two
children. One would like to see sleep at different ages and under
various conditions of fatigue similarly portrayed, for there is a range
and variety of expression in those who sleep, not perhaps as extensive,
but as beautiful as that to be found in those who are awake.
All things on the earth may be said (if we use the term in a wide sense)
to sleep, for all are affected by the stimulation to activity caused by
sunlight and by its cessation during night. It is only of late years
that we have come to know of fishes, crabs, worms, and star-fishes (many
of them without eyes) which live in the depths of the ocean, where no
light penetrates and it is always night. The ultimate source of their
food is in the upper sunlit layers of water, to which they never
penetrate, and from which particles of dead but nutritious matter (the
bodies of those who have lived up there) rain down upon them
incessantly, like manna on the Israelites. All things accessible to the
sun’s rays are not equally, nor even similarly, affected by the
alternation of day and night, and some not directly at all, but only by
the sleeping and waking of other things. The food of all living things
comes ultimately from plants which, in the presence of sunlight, and
only in that presence, and in virtue of its action upon their green
leaves, manufacture starch and sugar from the carbonic acid which exists
in the air and water around them, whilst they are also thus enabled to
take up nitrogen, and so to form their living substance or protoplasm.
At night those particles or cells of the living protoplasm of plants
which are furnished with transparent green granules, so as to entangle
the sunlight, and by its aid feed on carbonic acid, cease this work.
They necessarily repose from their labour because the light has gone.
This is the simplest example of the sleep of living things. And that
here, too, as in higher creatures, sleep is not a merely negative
thing—a mere cessation—is shown by the fact that it is at night that
other changes go on in the plant. The manufactured food takes effect on
the cells or particles nourished by it; in the night the well-fed,
enlarged “cells” in the growing parts of many plants slowly divide each
one into two, and each of these again into two, and so on, so as to
increase their total number and produce growth and development of the
plant. This alternation of activities in day and night occurs even in
the invisible microscopic vegetation of pools and streams. Animals—even
the most minute, only visible with a strong microscope—move about in
search of “bits” of food—in fact, bits of other animals or of
plants—and they, too, are, with special exceptions, checked in their
search for food by the darkness, for even extremely minute and simple
animals are guided in their search by light—that is to say, by a more
or less efficient sense of sight. Thus we see that in a general way the
sun is truly the ruler of life, and that when he is hidden from us we
all become quiescent, a condition which may be rightly considered as
the elementary form—the simplest equivalent of the sleep of man. The
quiescence which falls on the earth with the setting of the sun has,
however, become the opportunity of two different classes of living
things to seize an advantage. Beasts of prey, many of them, sleep during
the day, and steal forth at night on velvet foot to pounce on the
slumbering animals which are their necessary food. Another group of
timid animals, moths and small beasts like mice, hedgehogs, and lemurs,
find their safety in the dark, and only then venture forth. Even so, the
moths are met by special nocturnal enemies, the bats. So that the
primitive arrangement is complicated by a wakefulness, exchanging day
for night.
It is natural to apply the word “sleep” to the state of profound repose
which other living things appear to enter upon at night, so far as we
can judge by changes of activity and attitude—although it must be
remembered that the sleep of man is what we really indicate by that
word, and that it is difficult to trace anything beyond a superficial
similarity between man’s sleep and the repose or quiescence following
upon activity in other living things—excepting those which by their
structure and the working of their mechanism are obviously comparable to
man, such as beasts, birds, reptiles, and fishes. The “sleep of plants”
is the term applied to the closing of the flower, the drooping of the
flower-head and of the leaves of many of the common flowering plants,
which occurs at sunset or during the later hours of sunlight. But it
seems that this is not really comparable to man’s sleep. The closing of
the flower appears to be a protection of its perfume from useless
evaporation during the darkness, and the drooping a device to avoid the
settlement of dew and the injurious action of cold. Living things always
furnish us with examples of adaptations resisting the general law—and
as there are moths which fly by night, so also there are flowers which
remain closed by day and open at night to attract these moths, by whom
their pollen is carried and their fertilisation effected. The
tobacco-plants of our gardens are examples of these night-opening
flowers, which attract the nocturnal moths by their heavy perfume, and
there are many others.
The movements of plants are much more definite and varied than one is
apt to suppose. Leaves and flowers turn to or away from the sun, or to
or from the position which will favour a deposit of moisture; or, again,
their tendrils will explore and seize upon supports, enabling them to
secure a hold, and so to climb. The sensitive plant exhibits rapid
drooping movements of its leaflets and leaf-stalks when touched or
subjected to vibration.
An allied plant which shows slower but definite movement of its leaflets
has been supposed to furnish thereby prophetic indications of the
weather, and even to foretell earthquakes. This plant is the _Abrus
precatorius_, the seeds of which are called crab’s-eyes, and are used in
India by jewellers and druggists as weights—averaging a little less
than two grains. They are harmless when eaten, but contain a poison
called abrine, which causes them rapidly to produce fatal results when
introduced beneath the skin. Under the name “jequerity” they were
introduced into this country in 1882 for the treatment of ophthalmia.
This is the plant which was celebrated, about twenty years ago, as the
earthquake plant or weather plant, owing to the statements of an
Austrian naturalist as to its marvellous powers of prophecy by the
movement of its leaflets—statements which were carefully examined by
botanists at Kew Gardens at the time and shown to be devoid of
justification. Earth tremors, like other vibrations, cause the leaflets
to move and change their pose as they may cause animals to utter cries
of alarm, but the movements of the leaflets have no more prophetic
character than have those of the delicate pendulums, called
seismographs, by which it is now usual to register the constantly
occurring slight vibrations of the earth’s crust.
That beasts and birds enjoy a nocturnal sleep similar to that of man,
which is occasionally—like his sleep—transferred from night to
daytime, is a matter of common knowledge. These animals, like man, lower
the eyelids and adopt a position of ease when sleeping, even though they
often remain poised on their legs. The question has been raised as to
whether fishes sleep, since they have no eyelids and remain when at rest
poised in the water. We made some inquiries on this subject in the
laboratory of the Marine Biological Association at Plymouth some years
ago, and came to the conclusion, from the observation of various marine
fishes in the aquarium there, that fishes do sleep at night. They come
to rest on the bottom of the tanks, and are not so quickly responsive to
a touch or intrusion of any kind as they are in the daytime. It is
probable that this condition of repose is more definitely marked in some
kinds of fishes than in others, but in all shallow-water marine
organisms the absence of light produces a corresponding period of
quiescence. That there is a good deal more than this involved in the
sleep of the higher animals and of man will be apparent when we come to
study it more closely.
The sleep of man, and of animals which have, like man, a large and
well-developed nervous system—has for its salient feature the cessation
or extreme lowering of the “psychical” activity of the brain. When sleep
is at its height external agents (such as a touch, a sound, a flash of
light) which in the waking state set up through the nerves of the
organs of the senses complex changes in the brain, no longer do so. They
not only fail to excite consciousness and to leave their mark on the
memory, but they do not produce even a simple unconscious response. Yet
if they are of a sufficient degree of violence (varying according to the
depth of the sleep), they do reach the brain, and thus “awake” the
sleeper. Corresponding to the absence of receptive activity of the brain
in sleep is the absence of outgoing impulses from that organ; there is
no such control of the muscles as in the waking state, the head nods,
the eyelids droop, and the muscular action by which the erect posture is
maintained is in abeyance, although in a greatly lessened degree some
amount of muscular tone is unconsciously retained.
The passage from the waking state to that of deep sleep is not sudden
but graduated, and so is the process of awakening. In the intermediate
condition, either before or after deep sleep (often only a minute or two
in duration) the brain can still receive, more or less confusedly,
impressions from the exterior through the organs of sense, and it is in
this way that “dreams” are set going, and may be afterwards either
forgotten or remembered. In full sleep the mind is a blank. As a rule
healthy sleep becomes gradually more complete in the first hour, and
then very slowly less profound. But there are not any sufficient
observations on the “quality” of sleep after short or long duration. In
sleep it is not only the brain which is at rest: the whole body shares
in the condition. The pulse and breathing are slower, the digestive
organs and the bladder are more or less at rest. Both the intake of
oxygen into the lungs and the expiration of carbonic acid are lessened.
The chemical changes within the body are lessened though still
proceeding, and as a consequence the temperature is lowered.
It is curious how incomplete at present is the physiologist’s knowledge
of both the actual condition of the brain in sleep and of the immediate
causes which produce that condition. It is probably true (though it is
disputed) that the brain becomes pale during sleep, owing to a
contraction of the blood vessels, and that the inactivity of the brain
arises from this condition. But it is not obvious what determines the
contraction of these vessels at the definitely recurring period of
sleep. It is probable that the nervous tissue of the brain is, as are
the muscles of the body, poisoned or choked (as it were) by the chemical
products of the day’s activity, and so readily cease to be active until
the injurious products have had time to be carried away by the blood
stream. Muscular substance undoubtedly is affected in this way, and that
great muscle the heart, though never resting for a lengthened period,
rests after each pulse or contraction, and recovers itself in the brief
interval.
It is also probable that the exhaustion by the day’s activity of the
oxygen stored up in the various tissues of the body produces a condition
of quiescence whilst the store is replenished. Stimulation of the nerves
through the sense-organs of sight, hearing, and touch will prevent or
retard this natural quiescence, and the cessation of that stimulation is
favoured first of all by the darkness of night and by the closing of the
eyelid, as well as by the removal of clothes which more or less irritate
the skin; also by the would-be sleeper taking up a position of perfect
rest, and by the exercise of his will, withdrawing his brain as much as
possible from all external influences. The would-be sleeper also
controls, when possible, that internal stimulation of the brain which we
call attention. It is the failure (owing to unhealthy conditions) to
control the latter which leads to the most serious kind of
sleeplessness, when the brain gets for hours out of restraint and works
incessantly like an independent existence. The disturbance of the
nervous system set up by irritation of the digestive organs, whether
accompanied by pain or not, is an independent cause of sleeplessness
which often co-operates with the first, and is (through the mechanism of
the nerves) often set going (though it may arise independently) by an
unhealthy excess in the excitement of the brain’s activity. There is no
panacea for sleeplessness; the only thing to do is to consult a
first-rate physician, and strictly follow his advice.
There are many irregularities and abnormal manifestations of sleep.
There is the sleep which is induced by drugs such as opium, chloral, and
alcohol, and that induced by chloroform, ether, and nitric gas. There is
the heavy sleep accompanied by stertorous breathing, and there is the
unconscious condition called “coma.” Then there is the prolonged
sleeping called “trance,” of which that of the Sleeping Beauty, only to
be broken by a kiss, is an example. It is not possible, in the present
state of knowledge, to give an adequate account and explanation of the
condition of the brain in these different forms of sleep, nor of the
causes which induce that condition. One of the most interesting forms of
sleep is the condition called “somnambulism,” or sleep-walking, in which
part only of the brain is asleep, and other parts connected with various
degrees of mental activity are in waking order. Sleep-walking is a
condition which occurs spontaneously. On the other hand, “hypnotism” is
the name for a peculiar kind of sleep produced intentionally by an
operator on a patient by certain treatment and direction. In one of the
stages of artificially induced hypnotic or “mesmeric” sleep—called the
somnambulic stage—only so much of the brain is asleep as is concerned
with conscious memory. The brain receives stimulation through the
sense-organs, and the patient has the eyes open and appears to be
awake. In this state he is peculiarly open to suggestion by words, which
can be made to set up the most extraordinary illusions and consequent
behaviour. On “waking” the patient has no memory of what has occurred,
though a suggestion received in the somnambulic stage may persist in the
unconscious memory, and cause conduct on the part of the patient (many
hours after the brief hypnotic sleep has passed) which is entirely
inexplicable by the patient himself or by those who are not aware of the
fact that he had received a “suggestion” or “direction” when in the
hypnotised state. The senses of smell, hearing, and touch are often
abnormally acute in a hypnotised patient, but there is no evidence to
show that the brain of such a person can be influenced or “communicated
with” excepting through the ordinary channels of the sense-organs.
“Day-dreaming” and “reverie” are conditions resembling the hypnotic
sleep. The brain of each of us is constantly doing much of its work in a
state of partial hypnotism, and the term “unconscious cerebration” has
been used to describe it. A most interesting and difficult chapter of
the study of mental disease belongs here.
The prolonged sleep of some animals in the winter, called “hibernation,”
seems to be closely similar to ordinary sleep, but is set up by the
depressing action of continuous cold instead of by the daily recurring
quiescence of night and by the exhaustion due to the day’s activity.
Many animals—such as the marmot and dormouse, the frog and the
snail—exhibit this winter sleep. It has been found by experiment that
even in midsummer the dormouse can be made to “hibernate,” by exposing
it artificially to a low temperature, and hibernating animals can be
roused from their long sleep by bringing them into warmth. During the
winter sleep hibernating animals take no food, the pulse is slowed
down, and the body temperature falls. The scattered fat of the body, and
fatty matter and other material stored in special structures called
“hibernating glands,” are oxidised and slowly consumed during this
period, which may last for three or even four months. The animal on
waking is often in a very emaciated condition.
It is undoubtedly the case that the human natives of high latitudes
(such as the Norwegians), where there is no night in full summer, and
where there is prolonged darkness in winter, have acquired the habit of
keeping awake for many days in succession in summer, whilst making up
for the loss of sleep by excessive indulgence in it during the winter.
It is by no means clear how far man is capable of resisting the demand
for recurrent daily sleep without injury to health. Undoubtedly many men
are compelled by their avocations to sleep by day and wake by night. The
length and duration of “spells of sleep” and the power to sleep little
or not at all at one season, and almost uninterruptedly at another,
without injury to health, are matters of habit, occupation, and
circumstance. We have no ground for saying that every man “ought” to
sleep eight hours or more per diem, or, on the contrary, for insisting
that he should only sleep five or less. All depends on what he is doing
when he is awake, and what other people are doing (so as to disturb him)
when he is asleep; and we do not even know whether ten or twelve hours’
sleep would injure a man, were he able to take it, nor can we suggest
how it would injure him supposing it did not interfere with his feeding
and exercise.
As to quantities of sleep, there is the curious fact that the amount
habitually taken in the civilised communities of this part of the world
differs at different ages. Babies sleep a good part of the twenty-four
hours, and probably schoolboys and schoolgirls (under our present
conditions of life and work) ought to be given ten hours or more. Whilst
adult men sleep from six to eight or nine hours, it is a curious fact
that old people—not very old people, but those of sixty-five or
thereabouts—often find themselves unable to sleep more than four hours
at night, and take an hour or two in the daytime to make up for the
deficiency. I remember hearing Mr. Darwin state this as to himself to
his physician, Sir Andrew Clarke, who said it was very usual at his age,
and difficult to explain, since at a greater age, when a man is called
“very old,” a more or less continuous somnolent condition sets in. The
father of a great judicial dignitary of these days, himself a barrister
in large practice, when he was sixty years old would snatch fifteen or
twenty minutes’ sleep at any and every opportunity throughout the day,
even at the midday meal sometimes, so as altogether to disconcert those
who were with him, and he told me that he never slept more than four
hours at night, but got up and commenced work at four in the morning.
The cessation in early old age of the desire for more than half the
amount of sleep taken by younger men suggests that the regulating cause
of the number of hours which are needed for sleep may be simply and
directly the actual amount of work done by body and mind. This
imperceptibly becomes less as men grow older, and so less recuperative
sleep is necessary, though what work they do may be more effective and
better adjusted to its purpose when they have arrived at the condition
which is called “old age.”
We have seen that sleep in its widest sense comprises the simple
condition of quiescence brought about in even the minutest living things
by the recurring night, as well as the strangely elaborated varieties of
cessation of activity in the whole or parts of the brain of man and of
his body. Some of these cessations of activity naturally and
spontaneously occur in unsophisticated mankind, when darkness falls on
the earth at each succeeding evening. And it is hardly possible to doubt
that a tendency to periodic sleep has become fixed in the substance of
living things by the alternation of night and day—as well as in some
cases by the change of the seasons.
I must conclude these notes about sleep by relating a very curious case
of sleep, resembling the winter-sleep of higher animals, on the part of
a snail. This was the case of a desert snail from Egypt, which was
withdrawn into its shell, the mouth of the shell being closed with a
glistening film secreted by the snail, as is usual with snails in this
country in winter when they sleep. The desert snail in question was
affixed to a tablet of wood in a glass case in the natural history
department of the British Museum on March 25, 1846. On March 7, 1850,
that is four years afterwards, it was noticed by a visitor looking at
the case that the snail had emerged from his shell and discoloured the
paper around, but had again retired. So the officials unlocked the case
and removed the snail from the tablet and placed him in tepid water. He
rapidly and completely recovered, crawled about as a wide-awake snail
should, and sat for his portrait. This may be regarded as an instance of
unusually long sleep, natural to this species of snail, and related
probably to the frequently prolonged dryness of the snail’s
surroundings.