From the contemplative point of view, being lost in thoughts of any kind, pleasant or unpleasant, is analogous to being asleep and dreaming.
Waking Up: A Guide to Spirituality Without Religion
It is a 2014 book by Sam Harris. Harris discusses a wide range of topics including secular spirituality, the illusion of the self, psychedelics, and meditation. He attempts to show that a certain form of spirituality is integral to understanding the nature of the mind.