( B9 ) 



Noriiird slee|) is nol caused Itv diseaso, iieitliei', fo oiir know Icdn-e 

 at leasi, by iiitoxicalicni, coiiscMjiieiitlv il may l)c. caused Uy cessalion 

 or decrease of stiuiuli from llie surroujidings, aud in exauiiuiufi; tiiese 

 surroundings, we observe llie pei'iodicaily operating cause of sleep 

 in all nature, in 



The .^ettiiKj of the sun 



with which numerous stimuli either disappear or cease to operate. 

 The peculiar characteristic of sleep, the disturbed funi.Mions, may be 

 satisfactorily ex])lained bj the decrease of stimuli occasioned l»y the 

 setting of the sun. 



]\Iany functions of the Ii\ ing organism depend on sunlight, and 

 when sunlight dissappears, their intensity diminishes or they may 

 even cease altogether. 



The assimilation of plants, the search for nourishment by animals, 

 the receiving of stimuli by which psychical functions are origijuited, 

 all these are dependent on sunlight. 



The })henomena of sleep having been once recognised as symptoms 

 of decreased functions, all reseaiches for the species of animals in 

 which sleep begins, must necessarily remain fruitless, because most 

 functions of l)Oth plants and animals in general are subject to a 

 change, corresponding to the alternation of day and night. 



These stimuli which still continue, operating during sleep, partialh 

 entertain all functions, the psychical ones included, as we are made 

 to know by experience when dreaming. 



The want of slee]) in man is a quality iidierited from the animal, 

 and it does not appear so directly dependent on the selling of the 

 sun as is the case in vegetal and animal kingdom, only because 

 man continues his struggle for life with the aid of artificial light. 



In my opinion, the setting of the sun sufTices to ex|)lain the 

 periodicity of sleep, and going to sleep notwithstanding the excita- 

 tion of still extant i)Owerful stimuli, nnist be accounted for by 

 heredity, and I think the solution of the enigma mentioned by 

 Hermann, is found here. 



The sim[)licity of this answer to the question about the causation 

 of sleep, presenting itself as a matter of course and reminding one 

 of the e<^^ of Columbus, is only an apparent one, because the results 

 of years of psychological researciies have taught us to seek for this 

 causation outside the functions, j)hysiology having sought \ainh for 

 an explanation to be furnished by the functions themselves. 



The existence of night-animals may be explained in this way that 



