This is a question I get asked a lot. The following answer comes from my book 35 Questions About The Brain.
The brain is filled with billions of neurons, each acting like a tiny battery which periodically discharges its voltage. One neuron firing would look like a single spike in voltage. Several neurons firing in a sequence is a spike train, like dominoes knocking each other over. Multiple spike trains form regional patterns. Many neurons, spike trains and regions firing at the same time form complex patterns.
If each neuron fired independently, the overall pattern of activity would be random. But because the brain’s neurons often fire in sequences, the overall pattern can be observed and measured. These patterns of oscillating neural activity are call brain waves.
Brain waves vary in amplitude, frequency and phase. Phase indicates whether a neuron or neural circuit is firing or resetting itself. Although phase information can be useful for describing individual neurons, phase information isn’t that useful when looking at overall brain activity because they cancel each other out.
Amplitude is how much voltage is involved. Oddly, when you’re awake your amplitudes are small. The larger amplitude waves come when you are asleep.
Frequency is how often the firing occurs. As it turns out, amplitude and frequency seem to be negatively correlated. The higher the amplitude, the slower the waves. We have low amplitude but frequent firings when we are awake. When we are asleep, the brain fires less often but with bigger spikes.
Right now, you are awake and your brain is doing several things at once. There is no synchronicity in the pattern; every region is doing its own thing. This firing pattern is called beta wave activity. Beta waves are low amplitude waves (5-10 microvolts) that occur 12-30 times per second. Beta waves are like the choppy waves in the bay.
When you close your eyes to relax, you exhibit alpha waves. These are slower in frequency (8-12 per second) but larger (30-200 microvolts) and more synchronized across the brain. Alpha waves are like rolling ocean waves.
Theta waves occur in the transition during sleep from alpha waves to delta waves. They are slower in frequency (4-8 per second) but at the high end of amplitude for beta waves (10 microvolts). You use beta when awake, alpha when relaxed, and theta when daydreaming.
When you’re in deep sleep, you have more and more delta waves. They are the slowest brain waves (2-4 cycles per second) and have the highest voltage peaks (20-200 microvolts) You can’t get much slower than two2 cycles; one is very slow and zero means you’re brain dead. Delta waves are about as low as you can get.
These are the main four brain waves. There are some others you might hear about. Gamma waves are high frequency waves at about 40 Hz (cycles per second). They may have something to do with consciousness but are mostly still a mystery. The other oddity are the Mu waves. At 8-12 Hz, they are similar to alpha waves but might have something to do with mirror neurons.