The reason of this seems to be that, owing to the condition of the mind during sleep, the nature of the sensation is not clearly recognizable.

Even in the case of familiar external impressions, such as the sound of the striking of a clock, there appears to be wanting that simple process of reaction by which, in a waking condition of the attention, a sense-impression is instantly discriminated and classed.

In sleep, as in the artificially induced hypnotic condition, the slighter differences of quality among sensations are not clearly recognized.

The activity of the higher centres, which are concerned in the finer processes of discrimination and classification, being greatly reduced, the impression may be said to come before consciousness as something novel and unfamiliar.

And just as we saw that in waking life novel sensations agitate the mind, and so lead to an exaggerated mode of interpretation; so here we see that what is unfamiliar disturbs the mind, rendering it incapable of calm attention and just interpretation.
This failure to recognize the real nature of an impression is seen most conspicuously in the case of the organic sensations.