Answer: “Slow wave sleep” refers to the low frequency waveforms that are produced by the cortex while a person is in deep sleep.
When we sleep, the brain does not completely shut down. Instead, there continues to be electrical activity of neurons once we fall asleep. While we are awake, the neurons are often active in an asynchronous pattern. During this type of activity, they are sending signals back and forth in an unpredictable manner. However, once a person falls asleep, their brain activity will start to change. In deep sleep, large ensembles of brain cells will begin firing synchronously. The activity of these groups of neurons can be recorded by observing electrical activity across the scalp, as performed by an electroencephalogram, or EEG.
When a person falls asleep, they pass through stages of brain activity called non-REM sleep, each one characteristic of progressively deeper sleep. NREM1 is considered light sleep, NREM2 is an intermediate level of sleep, while NREM3 is deep sleep. During deep sleep, it is difficult to wake someone from their sleep. Each of these stages of NREM sleep have characteristic waveforms that appear in the EEG traces. (Previous definitions of sleep stages separated stage 3 from stage 4 sleep, but the current convention is to combine the two into a single stage of sleep.)
Purpose of deep sleep
Deep sleep is sometimes also called “restorative sleep”, since it is theorized the brain and the body undergoes increased cellular repair during slow wave activity. There is strong evidence suggesting this phase of sleep is important for healthy living, since the glymphatic system, a mechanism to “rinse out the brain”, becomes more active during deep sleep. In rodents, more activation of the glymphatic system has been shown to wash out proteins called amyloid beta. Amyloid beta has long been implicated as playing a role in Alzheimer’s disease.
However, these observations are not strongly replicated in humans (The impact of sleep components, quality and patterns on glymphatic system functioning in healthy adults: A systematic review).
EEG readings during deep sleep
The primary frequency that is observed during slow wave sleep is delta waves. Delta waves have a frequency range between 0.5 and 4 Hz, and are often high in amplitude. These are much slower than the types of waves that are observed at other points during the sleep / wake cycle. These delta waves are produced in response to the synchronous firing of large groups of neurons, and vary tremendously from the waveforms observed early in sleep. Because NREM3 is characterized by these slow events, it is sometimes called “slow wave sleep.”
Alpha waves (7-12 Hz) are apparent during wakefulness or focused brain activity. For example, if a person were performing mathematical calculations while awake, you might expect the alpha waves to predominate in the EEG trace.
Theta waves (6-10 Hz) are more prevalent during early, light sleep such as NREM1, as well as during NREM2.
Occasionally, a short burst of slow wave delta activity may occur early in NREM sleep, such as during NREM2. These bursts are called K-complexes. When a person senses some external stimulus while asleep, their brain might demonstrate a K-complex.