Numerous physiological researches on both man and the lower animals have shown that there is a marked fall in blood-pressure during sleep. Tarchanoff1 showed that a fall in aortic pressure of from 20 to 50 mm. mercury took place in young dogs during the early stages of sleep, and Howell2 noted a like fall in the blood-pressure of man.
Leonard Hill,3 although admitting that a fall in pressure took place, did not believe that this drop was any greater during sleep than that which has been demonstrated to occur as a result of simple rest combined with the prone posture. Brush and Fayerweather,4 however, showed definitely that this was not the case, but that the fall was concurrent with sleep, and that it was much greater than the drop which sometimes, but not invariably, takes place simply from rest or on lying down.
Howell (Textbook) brings
BROOKS H, CARROLL JH. A CLINICAL STUDY OF THE EFFECTS OF SLEEP AND REST ON BLOOD-PRESSURE. Arch Intern Med (Chic). 1912;X(2):97-102. doi:10.1001/archinte.1912.00060200026002