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.