We exhaust ourselves in three ways: We create fabrications like mental concepts. We exert a lot of effort. We create many objects or targets in our minds. These are the three things that really tire us. It is like an insect caught in a spider's web: the more agitated it becomes, the more tangled it gets in the web. This creates real suffering, real torment for the mind.

-- Nyoshul Khen Rinpoche