Physical and mental rest, however, being easily attained through a change of activities, cannot be entirely synonymous with sleep. Sleep takes place mainly while we are resting, although we know of cases when sleep sets in regardless of continued muscular activity, but sleep is not exactly "rest." We do not sleep because we need rest. In many cases we can or could rest very well, although in such cases sleep is an impossibility.
What then induces sleep? The certainty that we can for a time relax our watch on our environment; a feeling of perfect safety; the conscious or unconscious knowledge that no danger threatens us.
Our receptive contact with reality is attained through the action of our vagotonic nerves which, as stated before, upbuild the body and assure the continuance of the race. Our defensive contact, on the other hand is attained through our sympathetic nerves which interrupt all the activities which are not necessary for fight or flight. As long as some stimulus is interpreted by those nerves as indicating a possible danger, we cannot sleep, although we may, under the influence of terrifying fear, fall into unconsciousness.
A light flashed on our closed lids at night causes us to wake up because sympathetic activities bid us to prepare for an emergency. A light burning evenly in our bedroom and not too bright to cause physical pain, will, on the other hand, allow us to sleep soundly because the constant character of the stimulus does not cause us to expect any danger therefrom.
A mouse rustling a bit of paper will wake us up, but trains passing in front of our window at regular intervals, or the constant rumble of a neighbouring power house will not prove a disturbance as soon as our nerves have learnt to interpret those stimuli as harmless.
Conversation with a dull, witless person, unlikely to best us in debate, puts us to sleep. Argument with keen, sharp-minded people, who keep us on the defensive, may lead to sleeplessness for the rest of the night. A dull book in which nothing happens or is expected to happen, acts as a soporific; we cannot close our eyes before we know the dénouement of a thrilling piece of fiction.
In other words, monotony transforms itself into a symbol of safety. Safety does not require the muscular tension, the blood stream speed which the organism needs in order to cope with possible emergencies. We "let go" and no longer pay any close attention to our environment. We sleep.