Whether or not we shall accept the explanation that sleep is due to brain anemia is one of the questions that is still obscured in the physiologic psychology of the future. Sleep still maintains much of its mystery in spite of scientific advance. Dreams have not lost their occult character entirely, notwithstanding all the progress that has been made in psycho-psychics and the related sciences. One thing seems sure: dreams constitute a manifestation of intellectual life and require the exertion of a certain amount of brain force or mental energy. As far as possible, then, they are to be avoided. Every one knows how tired one is who arises after having dreamed much during the night and, on the contrary, how recuperative is dreamless sleep. It is well understood that the reason why an excess of sleep, instead of being restful to intellectual energy, is rather exhaustive, is that, after a certain amount of sleep, even though the body continues to be somnolent, the mind awakes and, in the midst of the undirected, rapidly varying mental excursions which follow, as much nerve force and mental energy is expended as would be necessary for the more continuous thinking of regular intellectual work.
PATHOLOGY OF DREAMS.. JAMA. 2007;298(4):468. doi:10.1001/jama.298.4.468