When someone is in a deep, dreamless sleep, they’re in delta. Next up, between 4 Hz and 7.9 Hz, is “theta,” which correlates to REM sleep, meditation, insight, and (as is often necessary for insight) the processing of novel incoming stimuli. Between 8 Hz and 13.9 Hz hovers “alpha,” the brain’s basic resting state. People in alpha are relaxed, calm, and lucid, but not really thinking. Beta sits between 14 Hz and 30 Hz, and signifies learning and concentration at the low end, fear and stress at the high.
