Many physiologists, not unnaturally desirous of upsetting what they regard as a gratuitous metaphysical hypothesis, have pronounced in favour of an absolutely dreamless or unconscious sleep.
From the physiological point of view, there is no mystery in a totally suspended mental activity.

On the other hand, there is much to be said on the opposite side, and perhaps it may be contended that the purely physiological evidence rather points to the conclusion that central activity, however diminished during sleep, always retains a minimum degree of intensity.

At least, one would be disposed to argue in this way from the analogy of the condition of the other functions of the organism during sleep.

Possibly this modicum of positive evidence may more than outweigh any slight presumption against the doctrine of unbroken mental activity drawn from the negative circumstance that we remember so little of our dream-life.[75] Such being the state of physiological knowledge respecting the immediate conditions of sleep, we cannot look for any certain information on the nature of that residual mode of cerebral activity which manifests itself subjectively in dreams.

It is evident, indeed, that this question can only be fully answered when the condition of the brain as a whole during sleep is understood.