Brain waves the five rhythms an EEG records

What are brain waves?

Brain waves are the rhythmic electrical activity of the brain as an EEG records it from the scalp. The electrodes pick up voltage changes of a few to about a hundred millionths of a volt (microvolts, µV). The signal comes mainly from the synaptic currents of large groups of cortical neurons working in step, not from single cells. Pyramidal neurons of the cortex are thought to produce most of it because they are aligned in parallel and fire together; deep structures such as the thalamus and the brain stem do not contribute directly.

An EEG never shows one wave at a time. Slow and fast rhythms run together, and the slower ones are larger: in a brain recording, power falls steadily as frequency rises, a so-called 1/f-like slope, and rhythmic peaks such as alpha stand out above it (He, 2014). Changes are also specific to each band. At the same moment one band can weaken while another grows, at the same spot on the head or at different spots (Pfurtscheller and Lopes da Silva, 1999).

So the names delta, theta, alpha, beta and gamma describe frequency ranges in a recording, not mental modes. Saying that a person is “in alpha” or “in beta” mixes the two: several bands are present at every moment, and what changes is their balance. A clinician reading an EEG first describes the dominant rhythm and then the other ongoing rhythms. Even the slope itself matters for research: a 2020 study in Nature Neuroscience showed that standard band-power analysis can confuse the rhythmic peaks with the arrhythmic background beneath them.

Types of brain waves and their frequencies

There are five main types of brain waves, named by frequency: delta, theta, alpha, beta and gamma. The table uses the ranges of the clinical EEG glossary of the International Federation of Clinical Neurophysiology (Kane and colleagues, 2017), which the chart on this page follows. Delta formally starts at 0.1 Hz, but in practice an ordinary EEG reads it from 0.5 Hz, because it does not record very slow shifts.

Amplitude falls in the same order. The adult alpha rhythm is mostly below 50 µV, often much higher in children, and fronto-central beta mostly below 30 µV. Gamma is the smallest and the hardest to see from the scalp: muscle tension and tiny eye movements produce signals in the same range, and in one study paralysing the muscles changed scalp power above 20 Hz by 10 to 200 times (Whitham and colleagues, 2007).

Brain waves also change with age. A baby's main resting rhythm at the back of the head first appears at 3–4 months and averages 5.3 Hz up to one year, 6.8 Hz at 2–3 years, 7.9 Hz at 4–5 years and 8.7 Hz at 6–7 years, against 9.9 Hz in adults aged 16–50 (Alcala-Zermeno and colleagues, 2025). In a study of 1,498 healthy people aged 6–86, alpha frequency rose through childhood and fell again in old age (Chiang and colleagues, 2011).

The interactive part above is schematic, not a real recording. It shows a moving trace for each band, slowed down four times and with heights relative to one another, a model EEG of six states from relaxed wakefulness to REM sleep with the share of each band, and a hypnogram of a typical night. Each band has its own page with the research in detail: delta waves, theta waves, alpha waves, beta waves and gamma waves.

Brain wave chart: why the frequencies differ between sources

Brain-wave charts disagree because the bands are agreed slices of a continuous spectrum, and researchers draw the borders in different places. A review of 184 resting-EEG studies (Newson and Thiagarajan, 2018) found that the most common definition of each band was used by only 30–50 % of the studies. Delta started anywhere from 0 to 2 Hz and ended between 3.5 and 6 Hz; beta started between 12 and 15 Hz and ended between 20 and 50 Hz.

The authors put the consequence plainly: what one publication means by delta or beta is not necessarily what another means. When you compare two articles, or two apps, check their band limits before you compare their numbers.

Bands are not identical from person to person either. In adults the alpha peak sits around 10 Hz on average (10.3 Hz in a study of 51 people, Haegens and colleagues, 2014), but it differs between people and speeds up a little with mental effort. For this reason some researchers anchor the bands to each person's own alpha frequency instead of fixed numbers (Klimesch, 1999).

What is an EEG and what does it measure?

An electroencephalogram (EEG) is a record of the brain's electrical activity taken with electrodes on the surface of the head. In clinics and research the electrodes are usually placed by the international 10–20 system: the head is measured between four landmarks, and the electrodes go at steps of 10 % or 20 % of those distances. A denser layout is called the 10–10 system.

Each electrode sees the summed activity of a very large number of neurons beneath it. Synaptic currents are the major contributor, while spikes, ion-channel currents and the cells' own membrane oscillations also shape the signal (Buzsáki, Anastassiou and Koch, 2012). Researchers are frank about the limits: a 2017 review in Trends in Neurosciences argued that “we know shockingly little” about exactly where EEG signals come from and what they mean, and that oscillations are the best anchor for linking them to brain circuits.

For about 40 years, from 1930 to 1970, the EEG was the main tool for locating brain lesions, until CT scanning arrived. Today a neurologist uses it to describe the background rhythm, look for epileptic activity and abnormal slowing, and score sleep. Slowing is an important sign: generalised delta activity outside deep sleep, or theta outside drowsiness and light sleep, is considered abnormal in an adult EEG, most often in encephalopathy, with causes that include brain injury, neurodegenerative disease, metabolic or toxic disturbances, infection and sedating drugs (Alcala-Zermeno and colleagues, 2025).

Who discovered brain waves? Hans Berger and the first EEG

The German psychiatrist Hans Berger recorded the first human EEG on 6 July 1924 in Jena, during an operation on a 17-year-old boy; strictly speaking it was a recording from the surface of the brain, an electrocorticogram. Before him, Richard Caton had recorded electrical currents from the exposed brains of animals. Berger reported his results in 1929, using the terms alpha and beta waves for the two rhythms he saw.

The publication was met with scepticism. Substantial work began only after Edgar Douglas Adrian demonstrated the rhythm publicly in 1934 (Borck, 2006). Through the 1930s the alpha rhythm was widely called the “Berger rhythm”, as the titles of papers from that time show, for example a 1936 article in Science on the temperature characteristics of the Berger rhythm in man. Delta, theta and gamma were named later, as EEG research spread in the 1930s and 1940s.

Berger's biography is darker than the old story. It was long said that the Nazis forced him out of his post. Historical research published in 2014 (Zeidman, Stone and Kondziella) shows that he helped choose his Nazi successor, gave money to the SS and sat on Nazi genetic health courts that reviewed forced sterilisations.

Brain waves during sleep: what each stage looks like

Each stage of sleep is defined by its brain waves. A sleep lab divides the night into 30-second epochs and scores each one as wake, N1, N2, N3 or REM under the rules of the American Academy of Sleep Medicine. The older system of 1968 had four non-REM stages; its stages 3 and 4 were merged into N3, also called slow-wave sleep.

Adults sleep in cycles of about 90–100 minutes, usually four to five a night. Most deep sleep comes in the first non-REM period; the first REM period arrives after about 90 minutes, is short, and REM periods grow longer towards morning (Steiger and Pawlowski, 2019). Shares change with age: a meta-analysis of 65 studies with 3,577 people aged 5–102 found that the share of slow-wave sleep falls steadily from childhood on (Ohayon and colleagues, 2004).

Deep sleep also answers to how long you were awake. After 40.5 hours without sleep, delta power in the first recovery night was clearly higher than normal (Borbély and colleagues, 1981, 8 young adults). This homeostatic sleep pressure, Process S, works together with the circadian clock, Process C. The delta waves page covers deep sleep in detail, and the theta waves page covers drowsiness and REM.

What are sleep spindles?

Sleep spindles are bursts of 11–16 Hz waves, most commonly 12–14 Hz, lasting at least half a second, and together with K-complexes they define stage N2 sleep. On a recording the waves grow and then fade, which gives the burst its spindle shape. Clinicians distinguish fast spindles of 12–15 Hz over the centre of the head from slow spindles of 9–12 Hz further forward. A K-complex is a large two-phase delta wave over the centre of the head, often with a spindle riding on it.

Spindles are generated in the thalamic reticular nucleus and synchronised by loops between the thalamus and the cortex. A 2020 review in Physiological Reviews calls them one of the most inheritable signatures of the sleep EEG (Fernandez and Lüthi). In more than 11,000 sleep recordings of people aged 4–97, spindle density rose through childhood, peaked around adolescence and declined across adult life (Purcell and colleagues, 2017).

Spindles are linked to learning. After an evening of intensive word-pair learning, people produced more spindles than after an equally demanding task without learning, most of all in the first 90 minutes of sleep, and those with more spindles remembered more (r = 0.56; Gais and colleagues, 2002). In a small 2007 study of 18 medicated patients, spindles were markedly weaker in schizophrenia than in healthy people or in people with a history of depression (Ferrarelli and colleagues).

What are mu waves?

Mu waves are a 7–11 Hz rhythm of arch-shaped waves over the central, motor strip of the head in an awake person. They overlap the alpha range in frequency but differ in place and in what blocks them: mu disappears when you move the opposite side of the body, think of moving, get ready to move or are touched, while alpha at the back of the head reacts mainly to the eyes opening and to attention.

Mu also weakens when you watch someone else move. A meta-analysis of 85 studies with 1,707 participants (Fox and colleagues, 2016) found that mu power fell during doing an action (d = 0.46) and, more mildly, during watching one (d = 0.31). This made the mu rhythm a common tool for studying the brain's “mirror” activity. In infants, a central 6–9 Hz rhythm is the forerunner of the adult mu rhythm (Marshall and colleagues, 2002).

Beta over the motor cortex behaves in a related way: it dips while you move and bounces back above its resting level within about a second after you stop, the so-called beta rebound (Pfurtscheller and colleagues, 1996). Both rhythms show that a band can carry very different meanings depending on where on the head it is recorded.

Brainwave entrainment: can sound or light change brain waves?

The brain does follow a rhythmic sound or flicker while it lasts, and this part is well measured. Play a sound that pulses about 40 times a second and the hearing brain answers with a stable rhythm at the same rate; such auditory steady-state responses can be recorded for rates from 1 to 200 Hz and are particularly prominent near 40 Hz in waking people (Picton and colleagues, 2003). Flickering light makes the visual cortex respond at the flicker rate from 1 Hz up to about 90 Hz, with the strongest responses near 10, 20, 40 and 80 Hz (Herrmann, 2001).

Whether that changes mood, sleep or thinking is a separate question. For binaural beats, a 2023 systematic review found 14 EEG studies: five found that the brain followed the beat, eight did not, and one was mixed (Ingendoh and colleagues). A 2019 meta-analysis of 22 studies found a medium-sized effect on memory, attention, anxiety and pain (g = 0.45; Garcia-Argibay and colleagues), which shows that people respond to the beats, not that the brain wave changed. The studies and a generator are on the binaural beats pages for sleep, meditation, anxiety, focus and 40 Hz.

Music works in a similar, limited way. The brain's slow rhythms lock onto the beat and the note rate of music, roughly 1–8 Hz, which the tempo sets (Doelling and Poeppel, 2015; Nozaradan and colleagues, 2011). No study has shown that a track labelled “alpha” or “delta” moves your brain into that band. Calm music may well relax you; the band name on it is marketing.

Flickering light carries a real risk. It can trigger seizures in people with photosensitive epilepsy: flash rates of 15–25 Hz are the most provocative, but anything from 1 to 65 Hz can do it, and about 1 in 4,000 people aged 5–24 is at risk (Fisher and colleagues, 2005). Do not use flicker devices if you or a family member has epilepsy or has had a seizure.

Schumann resonance and brain waves: is there a link?

The Schumann resonance is a real physical phenomenon: lightning around the world makes the space between the ground and the ionosphere ring at about 7.8 Hz, with overtones near 14, 20 and 26 Hz (Price, 2016). The German physicist Winfried Otto Schumann predicted these resonances in 1952, and Balser and Wagner first measured them in 1960. The frequency fluctuates; the popular figure of 7.83 Hz is a commonly quoted average of the first mode.

By the clinical glossary, 7.83 Hz falls in the theta band, just below the start of alpha at 8 Hz, and that numerical overlap has inspired many claims that the Earth tunes the human brain. The evidence is limited to correlational work, chiefly from one Canadian laboratory: in 238 EEG recordings from 184 people, peaks near the first three Schumann frequencies were discernible in averaged EEG “in some but not all participants” (Saroka, Vares and Persinger, 2016). Controlled studies have not shown that the resonance shapes brain activity. The site's daily page on the Schumann resonance today shows the current readings.

Can a consumer EEG headband measure your brain waves?

A consumer headband records real EEG, but from far fewer sites and with more noise than a medical system. The Muse headband, for example, has four sensors on the forehead and behind the ears, at positions close to AF7, AF8, TP9 and TP10. Researchers used it with 60 students to pick up standard brain responses such as the P300 (Krigolson and colleagues, 2017), while noting that it has no electrode at the back of the midline, where that response is normally largest.

In a direct comparison of two medical and two consumer systems in 5 people over two visits, Muse showed a broadband increase in power and the highest variation between visits, and the consumer devices were more prone to artefacts from blinks and muscle movement. The authors concluded that medical systems offer “clear advantages in data quality, reliability, and depth of analysis” (Ratti and colleagues, 2017). Consumer EEG is still widely used in research: a 2024 scoping review counted 916 studies, most of them on brain–computer interfaces (Sabio and colleagues).

A headband can show broad changes, such as more alpha when you close your eyes. It cannot read thoughts, diagnose anything or measure “calm” precisely, and a reading labelled “gamma” deserves the most caution, because muscle tension and eye movements fall in the same frequency range.