350 BC
ON SLEEP AND SLEEPLESSNESS
by Aristotle
translated by J. I. Beare
1

WITH regard to sleep and waking, we must consider what they are:
whether they are peculiar to soul or to body, or common to both; and
if common, to what part of soul or body they appertain: further,
from what cause it arises that they are attributes of animals, and
whether all animals share in them both, or some partake of the one
only, others of the other only, or some partake of neither and some of
both.
Further, in addition to these questions, we must also inquire what
the dream is, and from what cause sleepers sometimes dream, and
sometimes do not; or whether the truth is that sleepers always dream
but do not always remember (their dream); and if this occurs, what its
explanation is.
Again, [we must inquire] whether it is possible or not to foresee
the future (in dreams), and if it be possible, in what manner;
further, whether, supposing it possible, it extends only to things
to be accomplished by the agency of Man, or to those also of which the
cause lies in supra-human agency, and which result from the workings
of Nature, or of Spontaneity.
First, then, this much is clear, that waking and sleep appertain
to the same part of an animal, inasmuch as they are opposites, and
sleep is evidently a privation of waking. For contraries, in natural
as well as in all other matters, are seen always to present themselves
in the same subject, and to be affections of the same: examples
are-health and sickness, beauty and ugliness, strength and weakness,
sight and blindness, hearing and deafness. This is also clear from the
following considerations. The criterion by which we know the waking
person to be awake is identical with that by which we know the sleeper
to be asleep; for we assume that one who is exercising
sense-perception is awake, and that every one who is awake perceives
either some external movement or else some movement in his own
consciousness. If waking, then, consists in nothing else than the
exercise of sense-perception, the inference is clear, that the
organ, in virtue of which animals perceive, is that by which they
wake, when they are awake, or sleep, when they are awake, or sleep,
when they are asleep.
But since the exercise of sense-perception does not belong to soul
or body exclusively, then (since the subject of actuality is in
every case identical with that of potentiality, and what is called
sense-perception, as actuality, is a movement of the soul through
the body) it is clear that its affection is not an affection of soul
exclusively, and that a soulless body has not the potentiality of
perception. [Thus sleep and waking are not attributes of pure
intelligence, on the one hand, or of inanimate bodies, on the other.]
Now, whereas we have already elsewhere distinguished what are called
the parts of the soul, and whereas the nutrient is, in all living
bodies, capable of existing without the other parts, while none of the
others can exist without the nutrient; it is clear that sleep and
waking are not affections of such living things as partake only of
growth and decay, e.g. not of plants, because these have not the
faculty of sense-perception, whether or not this be capable of
separate existence; in its potentiality, indeed, and in its
relationships, it is separable.
Likewise it is clear that [of those which either sleep or wake]
there is no animal which is always awake or always asleep, but that
both these affections belong [alternately] to the same animals. For if
there be an animal not endued with sense-perception, it is
impossible that this should either sleep or wake; since both these are
affections of the activity of the primary faculty of sense-perception.
But it is equally impossible also that either of these two
affections should perpetually attach itself to the same animal, e.g.
that some species of animal should be always asleep or always awake,
without intermission; for all organs which have a natural function
must lose power when they work beyond the natural time-limit of
their working period; for instance, the eyes [must lose power] from
[too long continued] seeing, and must give it up; and so it is with
the hand and every other member which has a function. Now, if
sense-perception is the function of a special organ, this also, if
it continues perceiving beyond the appointed time-limit of its
continuous working period, will lose its power, and will do its work
no longer. Accordingly, if the waking period is determined by this
fact, that in it sense-perception is free; if in the case of some
contraries one of the two must be present, while in the case of others
this is not necessary; if waking is the contrary of sleeping, and
one of these two must be present to every animal: it must follow
that the state of sleeping is necessary. Finally, if such affection is
Sleep, and this is a state of powerlessness arising from excess of
waking, and excess of waking is in its origin sometimes morbid,
sometimes not, so that the powerlessness or dissolution of activity
will be so or not; it is inevitable that every creature which wakes
must also be capable of sleeping, since it is impossible that it
should continue actualizing its powers perpetually.
So, also, it is impossible for any animal to continue always
sleeping. For sleep is an affection of the organ of
sense-perception--a sort of tie or inhibition of function imposed on
it, so that every creature that sleeps must needs have the organ of
sense-perception. Now, that alone which is capable of sense-perception
in actuality has the faculty of sense-perception; but to realize
this faculty, in the proper and unqualified sense, is impossible while
one is asleep. All sleep, therefore, must be susceptible of awakening.
Accordingly, almost all other animals are clearly observed to
partake in sleep, whether they are aquatic, aerial, or terrestrial,
since fishes of all kinds, and molluscs, as well as all others which
have eyes, have been seen sleeping. 'Hard-eyed' creatures and
insects manifestly assume the posture of sleep; but the sleep of all
such creatures is of brief duration, so that often it might well
baffle one's observation to decide whether they sleep or not. Of
testaceous animals, on the contrary, no direct sensible evidence is as
yet forthcoming to determine whether they sleep, but if the above
reasoning be convincing to any one, he who follows it will admit
this [viz. that they do so.]
That, therefore, all animals sleep may be gathered from these
considerations. For an animal is defined as such by its possessing
sense-perception; and we assert that sleep is, in a certain way, an
inhibition of function, or, as it were, a tie, imposed on
sense-perception, while its loosening or remission constitutes the
being awake. But no plant can partake in either of these affections,
for without sense-perception there is neither sleeping nor waking. But
creatures which have sense-perception have likewise the feeling of
pain and pleasure, while those which have these have appetite as well;
but plants have none of these affections. A mark of this is that the
nutrient part does its own work better when (the animal) is asleep
than when it is awake. Nutrition and growth are then especially
promoted, a fact which implies that creatures do not need
sense-perception to assist these processes.

2

We must now proceed to inquire into the cause why one sleeps and
wakes, and into the particular nature of the sense-perception, or
sense-perceptions, if there be several, on which these affections
depend. Since, then, some animals possess all the modes of
sense-perception, and some not all, not, for example, sight, while all
possess touch and taste, except such animals as are imperfectly
developed, a class of which we have already treated in our work on the
soul; and since an animal when asleep is unable to exercise, in the
simple sense any particular sensory faculty whatever, it follows
that in the state called sleep the same affection must extend to all
the special senses; because, if it attaches itself to one of them
but not to another, then an animal while asleep may perceive with
the latter; but this is impossible.
Now, since every sense has something peculiar, and also something
common; peculiar, as, e.g. seeing is to the sense of sight, hearing to
the auditory sense, and so on with the other senses severally; while
all are accompanied by a common power, in virtue whereof a person
perceives that he sees or hears (for, assuredly, it is not by the
special sense of sight that one sees that he sees; and it is not by
mere taste, or sight, or both together that one discerns, and has
the faculty of discerning, that sweet things are different from
white things, but by a faculty connected in common with all the organs
of sense; for there is one sensory function, and the controlling
sensory faculty is one, though differing as a faculty of perception in
relation to each genus of sensibles, e.g. sound or colour); and
since this [common sensory activity] subsists in association chiefly
with the faculty of touch (for this can exist apart from all the other
organs of sense, but none of them can exist apart from it-a subject of
which we have treated in our speculations concerning the Soul); it
is therefore evident that waking and sleeping are an affection of this
[common and controlling organ of sense-perception]. This explains
why they belong to all animals, for touch [with which this common
organ is chiefly connected], alone, [is common] to all [animals].
For if sleeping were caused by the special senses having each and
all undergone some affection, it would be strange that these senses,
for which it is neither necessary nor in a manner possible to
realize their powers simultaneously, should necessarily all go idle
and become motionless simultaneously. For the contrary experience,
viz. that they should not go to rest altogether, would have been
more reasonably anticipated. But, according to the explanation just
given, all is quite clear regarding those also. For, when the sense
organ which controls all the others, and to which all the others are
tributary, has been in some way affected, that these others should
be all affected at the same time is inevitable, whereas, if one of the
tributaries becomes powerless, that the controlling organ should
also become powerless need in no wise follow.
It is indeed evident from many considerations that sleep does not
consist in the mere fact that the special senses do not function or
that one does not employ them; and that it does not consist merely
in an inability to exercise the sense-perceptions; for such is what
happens in cases of swooning. A swoon means just such impotence of
perception, and certain other cases of unconsciousness also are of
this nature. Moreover, persons who have the bloodvessels in the neck
compressed become insensible. But sleep supervenes when such
incapacity of exercise has neither arisen in some casual organ of
sense, nor from some chance cause, but when, as has been just
stated, it has its seat in the primary organ with which one
perceives objects in general. For when this has become powerless all
the other sensory organs also must lack power to perceive; but when
one of them has become powerless, it is not necessary for this also to
lose its power.
We must next state the cause to which it is due, and its quality
as an affection. Now, since there are several types of cause (for we
assign equally the 'final', the 'efficient', the 'material', and the
'formal' as causes), in the first place, then, as we assert that
Nature operates for the sake of an end, and that this end is a good;
and that to every creature which is endowed by nature with the power
to move, but cannot with pleasure to itself move always and
continuously, rest is necessary and beneficial; and since, taught by
experience, men apply to sleep this metaphorical term, calling it a
'rest' [from the strain of movement implied in sense-perception]: we
conclude that its end is the conservation of animals. But the waking
state is for an animal its highest end, since the exercise of
sense-perception or of thought is the highest end for all beings to
which either of these appertains; inasmuch as these are best, and
the highest end is what is best: whence it follows that sleep
belongs of necessity to each animal. I use the term 'necessity' in its
conditional sense, meaning that if an animal is to exist and have
its own proper nature, it must have certain endowments; and, if
these are to belong to it, certain others likewise must belong to it
[as their condition.]
The next question to be discussed is that of the kind of movement or
action, taking place within their bodies, from which the affection
of waking or sleeping arises in animals. Now, we must assume that
the causes of this affection in all other animals are identical
with, or analogous to, those which operate in sanguineous animals; and
that the causes operating in sanguineous animals generally are
identical with those operating in man. Hence we must consider the
entire subject in the light of these instances [afforded by
sanguineous animals, especially man]. Now, it has been definitely
settled already in another work that sense-perception in animals
originates ill the same part of the organism in which movement
originates. This locus of origination is one of three determinate
loci, viz. that which lies midway between the head and the abdomen.
This is sanguineous animals is the region of the heart; for all
sanguineous animals have a heart; and from this it is that both motion
and the controlling sense-perception originate. Now, as regards
movement, it is obvious that that of breathing and of the cooling
process generally takes its rise there; and it is with a view to the
conservation of the [due amount of] heat in this part that nature
has formed as she has both the animals which respire, and those
which cool themselves by moisture. Of this [cooling process] per se we
shall treat hereafter. In bloodless animals, and insects, and such
as do not respire, the 'connatural spirit' is seen alternately
puffed up and subsiding in the part which is in them analogous [to the
region of the heart in sanguineous animals]. This is clearly
observable in the holoptera [insects with undivided wings] as wasps
and bees; also in flies and such creatures. And since to move
anything, or do anything, is impossible without strength, and
holding the breath produces strength-in creatures which inhale, the
holding of that breath which comes from without, but, in creatures
which do not respire, of that which is connatural (which explains
why winged insects of the class holoptera, when they move, are
perceived to make a humming noise, due to the friction of the
connatural spirit colliding with the diaphragm); and since movement
is, in every animal, attended with some sense-perception, either
internal or external, in the primary organ of sense, [we conclude]
accordingly that if sleeping and waking are affections of this
organ, the place in which, or the organ in which, sleep and waking
originate, is self-evident [being that in which movement and
sense-perception originate, viz. the heart].
Some persons move in their sleep, and perform many acts like
waking acts, but not without a phantasm or an exercise of
sense-perception; for a dream is in a certain way a
sense-impression. But of them we have to speak later on. Why it is
that persons when aroused remember their dreams, but do not remember
these acts which are like waking acts, has been already explained in
the work 'Of Problems'.

3

The point for consideration next in order to the preceding
is:-What are the processes in which the affection of waking and
sleeping originates, and whence do they arise? Now, since it is when
it has sense-perception that an animal must first take food and
receive growth, and in all cases food in its ultimate form is, in
sanguineous animals, the natural substance blood, or, in bloodless
animals, that which is analogous to this; and since the veins are
the place of the blood, while the origin of these is the heart-an
assertion which is proved by anatomy-it is manifest that, when the
external nutriment enters the parts fitted for its reception, the
evaporation arising from it enters into the veins, and there,
undergoing a change, is converted into blood, and makes its way to
their source [the heart]. We have treated of all this when
discussing the subject of nutrition, but must here recapitulate what
was there said, in order that we may obtain a scientific view of the
beginnings of the process, and come to know what exactly happens to
the primary organ of sense-perception to account for the occurrence of
waking and sleep. For sleep, as has been shown, is not any given
impotence of the perceptive faculty; for unconsciousness, a certain
form of asphyxia, and swooning, all produce such impotence. Moreover
it is an established fact that some persons in a profound trance
have still had the imaginative faculty in play. This last point,
indeed, gives rise to a difficulty; for if it is conceivable that
one who had swooned should in this state fall asleep, the phantasm
also which then presented itself to his mind might be regarded as a
dream. Persons, too, who have fallen into a deep trance, and have come
to be regarded as dead, say many things while in this condition. The
same view, however, is to be taken of all these cases, [i.e. that they
are not cases of sleeping or dreaming].
As we observed above, sleep is not co-extensive with any and every
impotence of the perceptive faculty, but this affection is one which
arises from the evaporation attendant upon the process of nutrition.
The matter evaporated must be driven onwards to a certain point,
then turn back, and change its current to and fro, like a tide-race in
a narrow strait. Now, in every animal the hot naturally tends to
move [and carry other things] upwards, but when it has reached the
parts above [becoming cool], it turns back again, and moves
downwards in a mass. This explains why fits of drowsiness are
especially apt to come on after meals; for the matter, both the liquid
and the corporeal, which is borne upwards in a mass, is then of
considerable quantity. When, therefore, this comes to a stand it
weighs a person down and causes him to nod, but when it has actually
sunk downwards, and by its return has repulsed the hot, sleep comes
on, and the animal so affected is presently asleep. A confirmation
of this appears from considering the things which induce sleep; they
all, whether potable or edible, for instance poppy, mandragora,
wine, darnel, produce a heaviness in the head; and persons borne
down [by sleepiness] and nodding [drowsily] all seem affected in
this way, i.e. they are unable to lift up the head or the eye-lids.
And it is after meals especially that sleep comes on like this, for
the evaporation from the foods eaten is then copious. It also
follows certain forms of fatigue; for fatigue operates as a solvent,
and the dissolved matter acts, if not cold, like food prior to
digestion. Moreover, some kinds of illness have this same effect;
those arising from moist and hot secretions, as happens with
fever-patients and in cases of lethargy. Extreme youth also has this
effect; infants, for example, sleep a great deal, because of the
food being all borne upwards-a mark whereof appears in the
disproportionately large size of the upper parts compared with the
lower during infancy, which is due to the fact that growth
predominates in the direction of the former. Hence also they are
subject to epileptic seizures; for sleep is like epilepsy, and, in a
sense, actually is a seizure of this sort. Accordingly, the
beginning of this malady takes place with many during sleep, and their
subsequent habitual seizures occur in sleep, not in waking hours.
For when the spirit [evaporation] moves upwards in a volume, on its
return downwards it distends the veins, and forcibly compresses the
passage through which respiration is effected. This explains why wines
are not good for infants or for wet nurses (for it makes no
difference, doubtless, whether the infants themselves, or their
nurses, drink them), but such persons should drink them [if at all]
diluted with water and in small quantity. For wine is spirituous,
and of all wines the dark more so than any other. The upper parts,
in infants, are so filled with nutriment that within five months
[after birth] they do not even turn the neck [sc. to raise the
head]; for in them, as in persons deeply intoxicated, there is ever
a large quantity of moisture ascending. It is reasonable, too, to
think that this affection is the cause of the embryo's remaining at
rest in the womb at first. Also, as a general rule, persons whose
veins are inconspicuous, as well as those who are dwarf-like, or
have abnormally large heads, are addicted to sleep. For in the
former the veins are narrow, so that it is not easy for the moisture
to flow down through them; while in the case of dwarfs and those whose
heads are abnormally large, the impetus of the evaporation upwards
is excessive. Those [on the contrary] whose veins are large are,
thanks to the easy flow through the veins, not addicted to sleep,
unless, indeed, they labour under some other affection which
counteracts [this easy flow]. Nor are the 'atrabilious' addicted to
sleep, for in them the inward region is cooled so that the quantity of
evaporation in their case is not great. For this reason they have
large appetites, though spare and lean; for their bodily condition
is as if they derived no benefit from what they eat. The dark bile,
too, being itself naturally cold, cools also the nutrient tract, and
the other parts wheresoever such secretion is potentially present
[i.e. tends to be formed].
Hence it is plain from what has been said that sleep is a sort of
concentration, or natural recoil, of the hot matter inwards [towards
its centre], due to the cause above mentioned. Hence restless movement
is a marked feature in the case of a person when drowsy. But where
it [the heat in the upper and outer parts] begins to fail, he grows
cool, and owing to this cooling process his eye-lids droop.
Accordingly [in sleep] the upper and outward parts are cool, but the
inward and lower, i.e. the parts at the feet and in the interior of
the body, are hot.
Yet one might found a difficulty on the facts that sleep is most
oppressive in its onset after meals, and that wine, and other such
things, though they possess heating properties, are productive of
sleep, for it is not probable that sleep should be a process of
cooling while the things that cause sleeping are themselves hot. Is
the explanation of this, then, to be found in the fact that, as the
stomach when empty is hot, while replenishment cools it by the
movement it occasions, so the passages and tracts in the head are
cooled as the 'evaporation' ascends thither? Or, as those who have hot
water poured on them feel a sudden shiver of cold, just so in the case
before us, may it be that, when the hot substance ascends, the cold
rallying to meet it cools [the aforesaid parts] deprives their
native heat of all its power, and compels it to retire? Moreover, when
much food is taken, which [i.e. the nutrient evaporation from which]
the hot substance carries upwards, this latter, like a fire when fresh
logs are laid upon it, is itself cooled, until the food has been
digested.
For, as has been observed elsewhere, sleep comes on when the
corporeal element [in the 'evaporation'] conveyed upwards by the
hot, along the veins, to the head. But when that which has been thus
carried up can no longer ascend, but is too great in quantity [to do
so], it forces the hot back again and flows downwards. Hence it is
that men sink down [as they do in sleep] when the heat which tends
to keep them erect (man alone, among animals, being naturally erect)
is withdrawn; and this, when it befalls them, causes
unconsciousness, and afterwards phantasy.
Or are the solutions thus proposed barely conceivable accounts of
the refrigeration which takes place, while, as a matter of fact, the
region of the brain is, as stated elsewhere, the main determinant of
the matter? For the brain, or in creatures without a brain that
which corresponds to it, is of all parts of the body the coolest.
Therefore, as moisture turned into vapour by the sun's heat is, when
it has ascended to the upper regions, cooled by the coldness of the
latter, and becoming condensed, is carried downwards, and turned
into water once more; just so the excrementitious evaporation, when
carried up by the heat to the region of the brain, is condensed into a
'phlegm' (which explains why catarrhs are seen to proceed from the
head); while that evaporation which is nutrient and not unwholesome,
becoming condensed, descends and cools the hot. The tenuity or
narrowness of the veins about the brain itself contributes to its
being kept cool, and to its not readily admitting the evaporation.
This, then, is a sufficient explanation of the cooling which takes
place, despite the fact that the evaporation is exceedingly hot.
A person awakes from sleep when digestion is completed: when the
heat, which had been previously forced together in large quantity
within a small compass from out the surrounding part, has once more
prevailed, and when a separation has been effected between the more
corporeal and the purer blood. The finest and purest blood is that
contained in the head, while the thickest and most turbid is that in
the lower parts. The source of all the blood is, as has been stated
both here and elsewhere, the heart. Now of the chambers in the heart
the central communicates with each of the two others. Each of the
latter again acts as receiver from each, respectively, of the two
vessels, called the 'great' and the 'aorta'. It is in the central
chamber that the [above-mentioned] separation takes place. To go
into these matters in detail would, however, be more properly the
business of a different treatise from the present. Owing to the fact
that the blood formed after the assimilation of food is especially
in need of separation, sleep [then especially] occurs [and lasts]
until the purest part of this blood has been separated off into the
upper parts of the body, and the most turbid into the lower parts.
When this has taken place animals awake from sleep, being released
from the heaviness consequent on taking food. We have now stated the
cause of sleeping, viz. that it consists in the recoil by the
corporeal element, upborne by the connatural heat, in a mass upon
the primary sense-organ; we have also stated what sleep is, having
shown that it is a seizure of the primary sense-organ, rendering it
unable to actualize its powers; arising of necessity (for it is
impossible for an animal to exist if the conditions which render it an
animal be not fulfilled), i.e. for the sake of its conservation; since
remission of movement tends to the conservation of animals.

 

-THE END-

 

 

Aristotle

Philosophy & Classics

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