We all know that working on a difficult calculation has a different effect on our thinking than reading a novel. We notice that we get tired when our thinking demands an effort from us. This can be easily understood, since it provides us with a means to falling asleep more easily. It must, however, not be images that particularly irritate us, nor must it be thoughts that give us cause to worry; it must however be thoughts that we find difficult. This can be experienced by everyone: we fall asleep relatively easily when we fill ourselves with thoughts that awaken a feeling of duty in us.
Source (German): Rudolf Steiner – GA 143 – Erfahrungen des Übersinnlichen/Die drei Wege der Seele zu Christus – Winterthur, January 14, 1912 (page 31)
Translated bij Nesta Carsten-Krüger