Which brain wave is characterized by frequencies under 4 Hz?

Delta waves are characterized by their low frequency, typically under 4 Hz. They are predominantly associated with deep sleep and restorative processes in the body. During stages of deep sleep, the brain primarily produces delta waves, facilitating critical functions such as healing and growth.

In contrast, alpha waves range from about 8 to 12 Hz and are associated with relaxed, yet alert states, often observed when a person is awake but calm. Beta waves, with frequencies from 13 to 30 Hz, are linked to active thinking and problem-solving situations. Theta waves fall between 4 to 8 Hz and are often associated with light sleep, relaxation, and creativity. Thus, among the given options, delta waves clearly stand out as the brain waves with frequencies under 4 Hz.