Delta sleep

From Memory Alpha, the free Star Trek reference

Delta sleep is a state of very deep sleep without dreaming.

Delta sleep is induced by a delta-wave inducer. (DS9: "Invasive Procedures")

When Deanna Troi was tormented by the music Kevin Uxbridge implanted into her mind, Doctor Beverly Crusher suggested inducing delta sleep to tune out even the counselor's deepest dreams. (TNG: "The Survivors")