VIll] THE NERVOUS SYSTEM 105 



to awaken the sleeper. Thus it has been ascertained in man 

 that sleep is most intense during the first hour and a half (after 

 the first few minutes) and then becomes rapidl}^ less. Thus 

 sounds which fail to wake the sleeper during the early hours of 

 the night, will bring sleep to an end at any time from the beginning 

 of the third hour onwards. 



Apart from loss of consciousness, certain other physiological 

 phenomena characterise sleep. The respirations and the heart 

 beats become slower. Secretory activity, such as that of the 

 kidney, or of the mucous glands is diminished. It is believed 

 however that the digestive organs are little if at all less active 

 during sleep, provided that there is food in the stomach or 

 intestines. The supply of blood to the brain is said to be dimin- 

 ished during sleep, and this according to some is one of the main 

 factors inducing sleep. On the other hand, according to Howell, 

 the diminished supply of blood is of the nature of a result, the 

 cause being fatigue on the part of the vaso-motor centre m the 

 medulla. As a result of constant activity in the daytime this 

 centre is supposed to become so fatigued that it is no longer able 

 to maintain a sufficient flow of blood through the brain, and 

 unconsciousness or sleep is the result. 



Sleep is the period of rest and recuperation when the con- 

 structive or anabolic side of animal activity dominates over the 

 destructive or katabolic side. Want of sleep is even more 

 damaging than starvation, for dogs deprived of food for three 

 weeks may yet undergo recovery, whereas after five days without 

 sleep they die. But why the state of sleep should recur normally 

 with such rh\i:hmical regularity and what are the precise factors 

 in metabolism which govern this recurrence, are questions which 

 on the evidence available admit onl}' of very imperfect solutions. 

 That night is the time for sleep, both with man and with most 

 animals, is known to all, and it is obvious that the setting in 

 of darkness is its immediate cause, while the daylight which 

 comes at dawn is the stimulus for awakening. 



