PSYCHO-PHYSICAL PHENOMENA . 461 



its memory of dreams on awakening more vivid. Everything, in 

 fact, goes to show that sleep by day is lighter than by night, and 

 that Nature as Virgil says made the alternations of sleep and 

 waking to coincide with those of night and day. 



When we feel the need of sleep, we seek a convenient posture, 

 in which muscular relaxation is most complete. In consequence 

 of habit this need makes itself felt every day at the same hour. 

 But darkness, silence, or a monotonous sound, the incessant 

 shaking of a train, a dull lecture, the state of digestion, mental 

 fatigue, and lastly ennui and distaste for one's surroundings, all 

 induce sleep, and make the want of it felt even at unaccustomed 

 hours. 



Sleep is usually preceded by a sensation of pricking in the 

 conjunctiva and the cornea, due to dry ness from the cessation 

 of the lachrymal flow, by yawning, heaviness in the head, 

 weariness in the limbs, closing of the eyelids, asynergy of the 

 conjugated movements of the eyes, difficulty of focussing attention, 

 and finally by progressive dulling of the senses. In the state of 

 drowsiness that precedes sleep there is a sort of rhythm in 

 psychical activity, expressed in alternate concentration and dis- 

 traction. The former becomes shorter, the latter longer, till 

 consciousness is suspended. The initial liypnagogic hallucinations 

 coincide with the states of distraction or vacancy : they are 

 usually visual, like those described by Cardano, Goethe, and Joh. 

 Miiller, and become more and more vague, disconnected, and 

 void of sensorial elements, till they finally merge into a state of 

 absolute vacancy or unconsciousness. Sleep supervenes in one of 

 these states of hallucinatory distraction, which lasts barely 2-3 or 

 at most 5 minutes. 



The same rhythm of alternate states of attention and dis- 

 traction occurs in the drowsiness that precedes awakening. 

 Attention is at first very transient and then becomes more 

 concentrated, at the expense of the states of vacancy which are 

 gradually emptied of hypnagogic illusions. Awakening occurs at 

 the moment when this mental rhythm tends to disappear in the 

 normal course of the psychical process of waking up (Vaschide 

 and Vurpas). 



The duration of sleep varies with many extrinsic and intrinsic 

 conditions. New-born infants sleep 18-20 hours of the day; 

 adults on an average 8 hours ; old people only 5-6 hours. Women 

 usually sleep more than men. Convalescents sleep a great deal 

 after acute illnesses, and this also occurs at the beginning of 

 grave illness (Double). The influence of climate, as the effect of 

 hot and cold seasons, is uncertain. 



The depth of sleep varies from its commencement to its close. 

 Kohlschiitter (1862) first attempted to measure and construct its 

 curve at Fechner's suggestion. He employed auditory stimuli, on 



