delta wave
n.
A slow brain wave, having a frequency of fewer than six cycles per second, that emanates from the forward portion of the brain and is associated with deep sleep in normal adults. Also called delta rhythm.

[TahlilGaran] American Dictionary ▲

delta wave
n., informal, semi-technical A brain wave 1-3 cycles per second, associated with very deep sleep.
Good night, honey, I'm off to produce some delta waves.
Compare: CATCH SOME Z'S, HIT THE HAY or HIT THE SACK.

[TahlilGaran] English Idioms Dictionary ▲