When we have been thinking really intensely, we most easily fall asleep; and so if we cannot go to sleep, it is good to pick up a book, or occupy ourselves with something which requires concentrated thinking, study a book of mathematics, for instance. This will help us to fall asleep; but not something, on the other hand, in which we are deeply interested, such as a novel containing much that captivates our interest.
Here our emotions become aroused, and the life of the emotions is something that hinders us from falling asleep.When we go to bed with our feelings vividly stirred, when we know that we have burdened our soul with something or when there is a special joy in our heart which has not yet subsided, it frequently happens that we turn and toss in bed and are unable to fall asleep.
In other words, whereas concepts unaccompanied by emotions weary us, so that we easily fall asleep, precisely what strongly affects our feelings prevents us from falling asleep.
Source: Rudolf Steiner – GA 130 – Jeshu ben pandira – Lecture 1 – Leipzig, 4th November 1911