736 A TEXTBOOK OF PHYSIOLOGY 



fileep. From the third hour onwards the sleep in many cases is of a 

 light type. In some cases there is a second rise in the intensity of 

 the depth of sleep in the fourth to fifth hours after onset. This form 

 has been said to occur particularly in persons of a nervous tempera- 

 ment. Although the greatest refreshment from sleep occurs during 

 the first hours, it has been shown that the subsequent hours are also 

 effectual. It is easier to do complicated mental work after several 

 hours' sleep than after only a few hours' sleep, although simple mental 

 acts can be performed equally well after but a few hours' sleep. 



It is in the waking hours that dreams occur, and they are generally 

 of short duration. The dreams of healthy sleep are frequently of 

 the past or of a fantastic nature. Only in broken sleep is the day's 

 work continued or present-day affairs worried over. In healthy sleep 

 the parts of the brain most active during the day should rest most pro- 

 foundly during the night. The parts of the brain do not fall asleep 

 or wake at the same instant. Response to sound sensations are the 

 last to go and the first to return. 



In addition to the great brain, the other parts of the central 

 nervous system are also resting. The respiratory centre is affected. 

 Respiration is slowed and deepened, and frequently becomes some- 

 what periodic (Cheyne-Stokes) in type. The reflexes are depressed, 

 the knee-jerk is scarcely present or absent altogether. 



The rate of heart-beat is slowed, the heart beats less forcibly, and 

 the arterial pressure falls, during sleep. 



Owing to the relaxation of the muscles and of the bloodvessels in 

 the warm, quiet state of the body, the blood stagnates in the peripheral 

 capillaries and veins, and the circulation is much less rapid. Less 

 oxygen is required, and the blood moving slowly through the capil- 

 laries fulfils the metabolic needs of the resting tissues. It furnishes 

 a reserve supply of " munitions " for active service during the waking 

 period. 



In the brain, also, the venous side of the circulation is more con- 

 gested and the blood-flow sluggish. When the sleeper is aroused, 

 the tone of the muscles, skeletal and vascular, at once increases; 

 the blood is sent from the periphery to the viscera, and the velocity 

 of the blood-flow is increased through the brain. The general meta- 

 bolism is lessened during sleep. The carbon dioxide output and the 

 oxygen intake show a marked decrease. The bodily temperature also 

 falls to its lowest point during the night. In the early morning hours 

 the bodily metabolism is at its lowest ebb. 



The bodily secretions are diminished during sleep. The " night " 

 urine in healthy persons is less in quantity and more concentrated than 

 the day urine. The amount of saliva is also decreased, and possibly, 

 also, the alimentary juices. 



The onset of sleep is flavoured by muscular and mental fatigue 

 and by the withdrawal of all stimuli of the extero-ceptive nervous 

 mechanism, especially light and sound. 



Fatigue produces a metabolic condition of the nervous tissue 

 which tends to put in abeyance the power of response to stimulation; 



