Sleep is especially favourable to the production of emotion because while it allows a considerable amount of activity to sensory activities, and a very wide freedom to the imagery founded on sensory activities, it largely and in many directions inhibits motor activity. The actions suggested by sensory excitation cannot, therefore, be carried out. As soon as the impulse enters motor channels it is impeded, broken up, and scattered in a vain struggle. This process is transmitted to the brain as a wave of emotion.
Sometimes, indeed, as we know, motor co-ordinations, usually inhibited in sleep, are not so inhibited. The dreamer is able to execute, perfectly or imperfectly, some action which, really or in imagination, he desires to execute. He is then said to be in a state of somnambulism. The somnambulist, in the wide sense of the word, is not necessarily a person who walks in his sleep, but any person in whom a group of co-ordinated muscles is sufficiently awake to respond more or less adequately to the motor impulse from the sleeping brain. To talk in sleep is a form of somnambulism. When the motor channels are thus unimpeded, there is usually no memory of a dream on awaking. The impulses that reach consciousness can be, as it were, quickly and easily drained off to the surface of the nervous system, and they tend to leave no deep impress on consciousness.
'I worked late last night,' writes a lady, a novelist, 'went to bed, and dropped into a dead kind of sleep. When I woke this morning about seven a funny thing had happened. Two candles were burning in my room. When I went to bed I had only one burning, and I know I put that out. Now, there were two burning side by side as if I had been writing, and they had evidently been burning only an hour or so, I must have got up and lighted them in my sleep.'[76] The actions carried out in the somnambulistic condition are not usually co-ordinated with the action of higher emotions: thus, a young woman was impelled by a distended bladder, while still asleep, to get out of bed and proceeded to carry out the suggested action, but without further precautions, on to the floor; she was only awakened by an exclamation from her sister, who had been aroused by the sound. We seem to see that under a strong stimulus--unfinished work in one case, vesical tension in the other--the motor centres have awakened to activity in the early morning while the higher centres are still soundly asleep. If the second sleeper had not been awakened, in neither case would any memory of the incidents have remained.[77] There has been no struggle, and no resultant emotions have, therefore, been aroused to impress consciousness. It is evident that the lack of adaptation between sensory and motor activity is an important factor in dreams, and contributes to impart to them their emotional character.