SLEEP 161 



change their pose as they may cause animals to utter 

 cries of alarm, but the movements of the leaflets have no 

 more prophetic character than have those of the delicate 

 pendulums, called seismographs, by which it is now usual 

 to register the constantly occurring slight vibrations of 

 the earth's crust. 



That beasts and birds enjoy a nocturnal sleep similar 

 to that of man, which is occasionally like his sleep 

 transferred from night to daytime, is a matter of common 

 knowledge. These animals, like man, lower the eyelids 

 and adopt a position of ease when sleeping, even though 

 they often remain poised on their legs. The question 

 has been raised as to whether fishes sleep, since they 

 have no eyelids and remain when at rest poised in the 

 water. We made some inquiries on this subject in the 

 laboratory of the Marine Biological Association at Ply- 

 mouth some years ago, and came to the conclusion, from 

 the observation of various marine fishes in the aquarium 

 there, that fishes do sleep at night. They come to rest 

 on the bottom of the tanks, and are not so quickly 

 responsive to a touch or intrusion of any kind as they 

 are in the daytime. It is probable that this condition 

 of repose is more definitely marked in some kinds of 

 fishes than in others, but in all shallow-water marine 

 organisms the absence of light produces a corresponding 

 period of quiescence. That there is a good deal more 

 than this involved in the sleep of the higher animals and 

 of man will be apparent when we come to study it more 

 closely. 



The sleep of man, and of animals which have, like 

 man, a large and well-developed nervous system has 

 for its salient feature the cessation or extreme lowering 

 of the "psychical" activity of the brain. When sleep 

 is at its height external agents (such as a touch, a sound, 

 a flash of light) which in the waking state set up through 



