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On Sleep and Sleeplessness
By Aristotle

Translated by J. I. Beare

Part 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

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 withthe
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. 

Part 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 explainswhy
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'.

Part 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 thehead];
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

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. 



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