Sleep is a vital part of every day for most people, but what goes on when you close your eyes is often a mystery. Sleep studies, either for sleep research or to diagnose sleep disorders, open a window into what happens to your brain and body while you sleep. Sleep studies can break your sleep down into stages, with each stage typified by a certain type of brain activity. Delta waves are the slowest of the sleeping brain waves.

As you sleep, you progress through five stages in a cyclic pattern. You begin in stage one, light sleep, where your brain moves from the beta and alpha waves of wakefulness into theta waves of sleep. As you move from stage one to stage two, you continue to experience theta waves interspersed with periods of intense activity known as sleep spindles. When you pass in to the deeper sleep of stage three, delta waves become prominent and when they represent more than half your brain activity, you are in stage four, the deepest stage of sleep. After stage four you move backwards through the sleep stages and then enter the fifth stage, rapid eye movement, or dreaming sleep.

The brain activity measured during a sleep study is recorded on an electroencephalogram, or EEG, a device that uses sensors placed on the scalp to measure electrical activity in the brain. Brain activity is recorded as a line on a continuously scrolling piece of paper or computer screen. The line moves up and down as it registers electrical impulses and the result is a wave pattern whose shape, frequency and amplitude, or height, can be measured. Each type of brain wave is described using these three parameters.

Delta waves are slow waves associated with deep sleep. People wakened from deep sleep cannot recall any thoughts or dreams, so the purpose of delta waves is somewhat mysterious, but it has been hypothesized that it has a role in “resetting” the brain. Delta waves are more prominent in infants and young children, but occur in all people during sleep. Some sleep disorders such as sleepwalking and night terrors happen during delta wave sleep.

Delta waves have a frequency of less than 4 hertz, or 4 waves per second, and most occur between 0.5 hertz and 3.5 hertz. Stated another way, the duration of each delta wave is between one quarter of a second and two seconds. While delta waves are the slowest, they can also be thought of as the loudest waves. The amplitude or height of delta waves is 75 microvolts, the strongest electrical activity recorded in normal brain waves.