This is a restatement of Professor Salmon's thesis on the cause of sleep, published in French in 1910. The author's clinical observations during the intervening twenty years have only strengthened his conviction that sleep and wakefulness are regulated by the secretory activity of the hypophysis. The nutritive state of the nerve cells concerned in sleep depends on the antitoxic action of the secretion of the anterior lobe and is affected by the vasoconstrictor properties of secretion of the posterior lobe. A large portion of the book is devoted to a discussion of the disorders of sleep (hypersomnia, narcolepsy, dissomnia, hyposomnia, insomnia) and their treatment. Thus, twenty-two groups of pathologic conditions leading to hypersomnia are taken up in some detail. The book should prove of considerable value to the student of sleep and its disorders.