In order to arrive at the solution of this question, Professor Bernheim allowed many of his patients to 'have their sleep out.' He found its average duration was three or four hours, but, as with natural sleep, it varied with individuals and circumstances.

On several occasions it lasted fifteen, and once eighteen, hours. In all cases the patients awoke fresh and comfortable. Dr. Wetterstrand and others, as has been explained, often keep their patients in a state of hypnotic somnambulism for weeks at a time, with remarkable curative results.