Recent physiological observation has gone to prove that during sleep all the activities of the organism are appreciably lowered.
Thus, for example, according to Testa, the pulse falls by about one-fifth.

This lowering of the organic functions appears, under ordinary circumstances, to increase towards midnight, after which there is a gradual rising.
The nervous system shares in this general depression of the vital activities.

The circulation being slower, the process of reparation and nutrition of the nerves is retarded, and so their degree of excitability diminished.

This is clearly seen in the condition of the peripheral regions of the nervous system, including the sense-organs, which appear to be but very slightly acted on by their customary stimuli.
The nervous centres must participate in this lethargy of the system.

In other words, the activity of the central substance is lowered, and the result of this is plainly seen in what is usually thought of as the characteristic feature of sleep, namely, a transition from vigorous mental activity or intense and clear consciousness, to comparative inactivity or faint and obscure consciousness.