All kinds of stimuli may affect the sleeping individual and produce certain physiological effects with out awakening him ; they may cause movements, changes in res piration, dilation of the pupil, considerable changes in the cardio vascular system, and also they may provoke dreams. Measure ments of the volume of a limb show that, during the application of various stimuli, the limb diminishes in volume more or less in proportion to the intensity of the stimulus. The probable explanation of this is that the sensory stimuli act reflexly upon the vasomotor centre in the medulla, and in this way cause con traction of the blood vessels. Thus Mosso found that a strong stimulus to the skin or to a sense organ—but not strong enough to awaken the sleeper—caused a contraction of the vessels of the forearm, an increase of blood pressure, and an increased flow of blood towards the brain. So sensitive is the whole organism in this respect, even during sleep, that a loudly spoken word, a sound, a touch, the action of light or any moderate sensory impression modified the rhythm of respiration, determined a contraction of the vessels of the forearm, increased the general pressure of the blood, caused an increased flow to the brain, and quickened the frequency of the beats of the heart. These observations show how a physiological explanation can be suggested of the influence of external impressions in modifying the dreams of a sleeper. Fur ther, Mosso found that during very profound sleep these oscilla tions disappear : the pulsatory movements are uniform and are not affected by sensory impressions, and probably this condition exists when there is the unconsciousness of a "dead" sleep.
The intensity of sleep has been measured by the intensity of sound necessary to awaken the sleeper. Kohlschiitter used for this purpose a pendulum falling against a metal plate. At intervals of half an hour during the period of sleep, the auditory stimuli produced in this way were increased in intensity until waking was caused. According to these observations, the greatest intensity is reached about an hour after the beginning of sleep. From the second or third hour onwards, the depth of sleep is very slight, the activities of the brain being just below the threshold of alertness. The period from the third hour to the moment of spontaneous awakening is, presumably, as important for the restoration of the brain to its normal waking condition as the deeper period of the first two–three hours. The recuperative power of sleep, therefore, does not appear to be proportional to its intensity. The change in the intensity of sleep varies greatly in different individuals. In many, especially in children, there is a second period of slightly increased intensity of sleep between the fourth and fifth hour. In children of four years with a normal period of sleep of about twelve hours, there is a marked increase in intensity about an hour before the awakening.
Theories of Sleep.—Confining ourselves to the more recent attempts to explain the cause of sleep, we have the following : I. The Accumulation of the Acid Products of Metabolism.—It has been suggested that this accumulation took place during the period of activity and led to a diminution of the irritability of the central nervous system.
2. Consumption of the Intramolecular Oxygen.—According to this theory, the cells are able to store oxygen (intramolecular oxygen), which is used during activity more rapidly than it is formed, thus causing a state of partial anoxaemia. During sleep the intramolecular oxygen, that is the oxygen which is combined with the protoplasm to form the irritable living matter, is again replenished.
3. Toxin Theory.—It has been supposed that a special toxin, which might be called hypnotoxin, is formed during the waking hours and finally accumulates to such an extent that it diminishes the irritability of the central nervous system.
4. Neuron Theory.—This theory explains sleep as due to a tem porary retraction of the dendrite processes of nerve cells which leads to a disruption of the continuity of the nerve paths in the central nervous system. All these theories have now been prac tically abandoned for lack of direct experimental evidence.
5. Inhibitory Theory.—During recent years a vast amount of experimental material relating to the genesis and intimate nature of sleep has been brought forward by Pavlov. Together with his co-workers, Pavlov has studied in animals reflexes which involve the co-operation of the higher nervous centres, i.e., of the cortex of the hemispheres. For a description of this important branch of physiology, the reader is referred to the section on Brain and to original treatises.
Conditioned reflexes are based on the associative function of the brain. To give a simple example: if the feeding of an animal is repeatedly accompanied by some stimulus, e.g., a note or other sound, some visual, olfactory, tactile or in fact any stimulus whatever, it is observed that when the stimulus is applied alone it evokes the same effect as feeding, namely the appropriate muscular and secretory responses. The magnitude of these re sponses depends on many conditions, one of which is the intensity of the accompanying stimulus. A strong sound, after it has been associated with feeding, when applied alone evokes a stronger secretory and motor response than does a subdued sound under the same conditions. Conditioned reflexes have been successfully employed for the purpose of determining the scope of various perceptions of animals, for instance the upper and lower limits of audition, colour vision, etc., and also for the determination of the precision in discrimination between closely resembling stimuli. If, for instance, one note is always accompanied by feeding while another is not, it will be found that only the first note will evoke the reflex. The second will have no visible effect.