nter, the contraction of the vessels in the cortex, and finally
the rest sensation form together the complete circle. The difficulty
which arises in this case lies only in the fact that the cortex gone to
sleep annihilates also, of course, the fatigue sensation and the rest
sensation. For that reason the real circle can appear only in the
preparatory stages of sleep. As soon as sleep itself sets in, the circle
is broken. The circle character of every instinct must lead the physical
effect upward to a higher and higher degree. Not to become excessive,
the physical effect must be checked somehow. In all other spheres, it
finds its end in satisfaction, for instance, by eating or by the sexual
act. In sleep the circular process ends automatically by its own effect
as soon as complete sleep is reached. Its causes, the fatigue and the
rest feeling, are stopped, as soon as the effect, the anaemia, is
secured.

We see now how widely different starting points can lead to sleep and
can understand from it how widely different disturbances can prevent
sleep. Sleep must result when fatigue is coming, but sleep must also
result when the elements of the rest feeling are produced, and as we saw
that the components of the rest feeling were the sensations of decreased
sensitiveness and decreased activity, sleep must result when either the
sensations and associations are absent and actions are suppressed, or
when monotonous sensations and automatic actions raise the threshold.
Sleep must arise further if our will associates the mere idea of such
rest, and finally physical or chemical means may produce a sleep
bringing effect either on the lower center or on the blood-vessels and
cells of the cortex. Correspondingly sleep may be prevented by
disturbances in any one of these spheres. There may be no normal
fatigue, there may be no fatigue sensation, there may be no rest feeling
on account of perceptions, or on account of associations, or on account
of impulses to action; there may be no normal response in the
subcortical center, there may be no physical effect in the cortex on
account of an existing hyperaemia or on account of an abnormal condition
of the cells. The psychotherapeutic treatment must carefully analyze
which element would be fit to supply the last link in the circular
chain. Sometimes we need the suggestion of fatigue, sometimes the
inhibition of ideas, sometimes the suppression of impulses, sometimes
the suggestion of rest, a