Meanwhile we must be content with vague hypotheses.
It may be said, for one thing, that during sleep the nervous substance as a whole is less irritable than during waking hours.

That is to say, a greater amount of stimulus is needed to produce any conscious result.[76] This appears plainly enough in the case of the peripheral sense-organs.

Although these are not, as it is often supposed, wholly inactive during sleep, they certainly require a more potent external stimulus to rouse them to action.

And what applies to the peripheral regions applies to the centres.

In truth, it is clearly impossible to distinguish between the diminished irritability of the peripheral and that of the central structures.
At first sight it seems contradictory to the above to say that stimuli which have little effect on the centres of consciousness during waking life produce an appreciable result in sleep.