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ON SLEEP AND SLEEPLESSNESS ................................................................................................................1

by Aristotle..............................................................................................................................................1

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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

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 suprahuman agency, and which result from the

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

arehealth 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 senseperception 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 senseperception, 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 senseperception 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 senseperception, 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 senseperception, 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 senseperception, it is impossible that this should either sleep or wake; since both

these are affections of the activity of the primary faculty of senseperception. 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 timelimit 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 senseperception is the function of a special

organ, this also, if it continues perceiving beyond the appointed timelimit 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 senseperception 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

So, also, it is impossible for any animal to continue always sleeping. For sleep is an affection of the organ of

senseperceptiona sort of tie or inhibition of function imposed on it, so that every creature that sleeps must

needs have the organ of senseperception. Now, that alone which is capable of senseperception in actuality

has the faculty of senseperception; 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. 'Hardeyed'

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

That, therefore, all animals sleep may be gathered from these considerations. For an animal is defined as such

by its possessing senseperception; and we assert that sleep is, in a certain way, an inhibition of function, or,

as it were, a tie, imposed on senseperception, while its loosening or remission constitutes the being awake.

But no plant can partake in either of these affections, for without senseperception there is neither sleeping

nor waking. But creatures which have senseperception 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 senseperception to

We must now proceed to inquire into the cause why one sleeps and wakes, and into the particular nature of

the senseperception, or senseperceptions, if there be several, on which these affections depend. Since, then,

some animals possess all the modes of senseperception, 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

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 ita 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

senseperception]. This explains why they belong to all animals, for touch [with which this common organ is

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 senseperceptions; 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 senseperception]: 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 senseperception 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

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 senseperception 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 senseperception 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 strengthin 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 senseperception, 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 selfevident [being that in which movement and senseperception originate,

Some persons move in their sleep, and perform many acts like waking acts, but not without a phantasm or an

exercise of senseperception; for a dream is in a certain way a senseimpression. 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'.

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 senseperception 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 heartan assertion which is proved by

anatomyit 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 senseperception 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,

As we observed above, sleep is not coextensive 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

tiderace 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 eyelids. 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 feverpatients 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 upwardsa 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 dwarflike, 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

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 eyelids 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

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

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,

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

[abovementioned] 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 senseorgan; we have also

stated what sleep is, having shown that it is a seizure of the primary senseorgan, 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