delta rhythm 
noun [mass noun]
Electrical activity of the brain at a frequency of around 1-8 Hz, typical of sleep. The resulting oscillations, detected using an electroencephalograph, are called delta waves.Research has shown alpha wave intrusion into non-REM delta wave sleep 70% greater than controls....
- Brain research has shown that using sound phasing for brain synchrony will adjust the brain waves to deeper states down to the delta wave pattern.
- By stage 4, the brain produces delta waves almost exclusively.