From wikipedia:

John Lennon wrote Tomorrow Never Knows in January 1966, with lyrics adapted from the book The Psychedelic Experience: A Manual Based on the Tibetan Book of the Dead by Timothy Leary, Richard Alpert, and Ralph Metzner, which in turn was adapted from the Tibetan Book of the Dead. George Harrison and Paul McCartney confirmed this, stating that they and Lennon had visited the newly opened Indica bookshop — Lennon was looking for a copy of The Portable Nietzsche — and Lennon had found a copy of The Psychedelic Experience that contained the lines: “When in doubt, relax, turn off your mind, float downstream”.[7]

Lennon bought the book, went home, took LSD, and followed the instructions exactly as stated in the book.[8][9] The book held that the “ego death” experienced under the influence of LSD and other psychedelic drugs is essentially similar to the dying process and requires similar guidance.[10][11]