formation.

Sleep[1] the mysterious, the death in life which we all seek each
night, is likewise regulated by habit. Arising from the need of
relief from consciousness and bodily exertion, the mechanism of
sleep is still not well understood. Is there a toxic influence at
work? is the body poisoned by itself, as it were, as has been
postulated; is there a toxin of fatigue, or is there a
"vaso-motor" reaction, a shift of the blood supply causing a
cerebral anaemia and thus creating the "sleepy" feeling? The
capacity to sleep is a factor of great importance and we shall
deal with it later under a separate heading as part of the
mechanism of success and failure. At present we shall simply
point out that each person builds up a set of habits regarding
sleep,--as to hour, kind of place, warmth, companionship,
ventilation and even the side of the body he shall lie on, and
that a change in these preliminary matters is often attended by
insomnia. Moreover, a change from the habitual in the general
conduct of life--a new city or town, a strange bed, a disturbance
in the moods and emotions--may upset the sleep capacity. Those in
whom excitement persists, or whose emotions are persistent,
become easily burdened with the dreaded insomnia. Sleep is
dependent on an exclusion of excitement and exciting influences.
If, however, exciting influences become habitual they lose their
power over the organism and then the individual can sleep on a
battle field, in a boiler factory, or almost anywhere.
Conversely, many a New Yorker is lulled to sleep by the roar of
the great city who, finds that the quiet of the country keeps him
awake.