Definitions for "Delta waves"
These are dominant waves of deep, dreamless sleep, which take place in stages three and four of sleep.
brain waves with a high amplitude and a frequency of less than 4 cycles per second that are produced during deep sleep
these brain waves are of the greatest amplitude, but slowest wavelength, and typically occur around a frequency range of 1.5 to 4 cycles per second. Deep dreamless sleep is associated with the lower end of the delta wave frequency range. When we go to bed and read a book for a short while we are usually in a low beta wave state, but upon putting the book down and turning off the lights our brainwaves slow from beta, through alpha and theta, and finally upon falling asleep are in a delta wave state. People dream in 90 minute cycles when the delta wave frequencies increase into the frequency range of theta waves, active dreaming takes place as well as rapid eye movements.
(See Electroencephalography)