The sleep of ignorance, which we call “deep sleep,” is a great darkness. It feels like a darkness thousands of years old, and it is even older: it is the essence of ignorance, the root of samsara. No matter how many nights we sleep, every night for thirty years or seventy, we cannot finish sleeping. We return to it again and again as if it recharges us, and it does.

Ignorance is the sustenance of samsara and, as samsaric beings, when we dissolve into the sleep of ignorance, our samsaric lives are fed. We wake stronger, our samaric existence refreshed. This is “great ignorance” bnecause it is immeasurable.

We experience the sleep of ignorance as a void or blank, in which there is no sense of self and no consciousness. Think of a long, tiring day, rainy weather, a heavy dinner, and the resultant sleep in which there is neither clarity nor sense of self. We disappear. One manifestation of ignorance in the mind is the mental drowsiness that pulls us toward such dissolution in unconsciousness.

Innate ignorance is the primary cause of sleep. The necessary secondary causes and conditions for its manifestation are tied to the body and the body’s weariness.