The cause of this condition of the centres is supposed to be the same as that of the torpidity of all the other organs in sleep, namely, the retardation of the circulation.

But, though there is no doubt as to this, the question of the proximate physiological conditions of sleep is still far from being settled.

Whether during sleep the blood-vessels of the brain are fuller or less full than during waking, is still a moot point.

Also the qualitative condition of the blood in the cerebral vessels is still a matter of discussion.[72] Since the effect of sleep is to lower central activity, the question naturally occurs whether the nervous centres are ever rendered inactive to such an extent as to interrupt the continuity of our conscious life.
This question has been discussed from the point of view of the metaphysician, of the psychologist, and of the physiologist, and in no case is perfect unanimity to be found.

The metaphysical question, whether the soul as a spiritual substance is capable of being wholly inactive, or whether it is not in what seem the moments of profoundest unconsciousness partially awake--the question so warmly discussed by the Cartesians, Leibnitz, etc .-- need not detain us here.
Of more interest to us are the psychological and the physiological discussions.