Delta waves are low-frequency, high-amplitude waveforms that herald the arrival of the deepest type of non-REM sleep, stage 4 sleep. It is during this stage of sleep that somnambulism (sleepwalking) occurs. Alpha waves are characteristic of relaxed wakefulness. Beta-like activity is characteristic of either ale wakefulness or REM sleep. Sleep spindles and K-complexes are characteristic of stage 2 sleep, which is a deeper sleep that occupies roughly 45% of the sleep cycle. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 14. Electrical Activity of the Brain, Sleep--Wake States, & Circadian Rhythms. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.