Sleep is a mysterious thing. One moment you’re lying in bed and staring at the ceiling, the next you’ve nodded off and your body and mind are undergoing this powerful, restorative process. Lots of things happen over the next couple of hours, including cell regeneration, muscular repair and memory consolidation.

But perhaps one of the most fascinating changes that occurs during sleep is the shift in our brainwaves. A form of electrical activity within the brain – the neurons communicating with one another – our brainwaves vary in frequency depending on our state of consciousness: when we’re awake, their frequency is high, whereas things slow down when we drift off. Three distinct types of brainwaves occur during the sleep cycle, all of which correspond to different sleep ‘stages’. 

While understanding the different types of brainwaves might not help you get a better night’s sleep, it’s a fascinating insight into how the brain works and the differences between each sleep stage. So, to learn more, we asked Dr Matthew Walker, a professor of neuroscience and psychology at the University of California, Berkeley, and Oura medical advisor, to talk us through the different sleep brainwaves and what they mean. 

During a typical night’s sleep, the brain’s activity fluctuates. The first three stages of sleep are characterised by three types of brainwaves: alpha waves, theta waves and delta waves. The former are typically recorded when we’re in the process of falling asleep, whereas the latter two come about during sleep.

Once we pass into the REM or rapid-eye-movement stage, our brainwaves speed up and reflect those that are present when we’re awake, which is why this is the stage when most dreaming occurs. Your brain’s active state during REM sleep also explains why you’re more likely to remember dreams when you wake up in the middle of an REM period – your brain is already functioning at the same frequency as when you’re awake.

For now, however, let’s take a closer look at the slow brainwaves that arise as we sleep.

What are alpha waves?

When you’re in a chilled-out, relaxed state – eg when you’re relaxing in bed before you turn out the lights – your brain will be home to alpha waves.

“These waves are associated with calm, wakeful states, such as when we’re quietly reflecting or meditating,” Dr Walker explains.  

“They are slower in frequency than the beta waves that dominate our normal waking state when we are alert and focused, and act as a kind of ‘gateway’ to sleep; they signal a decrease in the functional engagement of the brain with the external environment and are often the last step before we drift off into the non-REM sleep stages.”

In terms of numbers, alpha waves tend to have a frequency of between eight and 12.99hz (8-12.99 waves per second). When you’re relaxed, you’ll be at the lower end of that spectrum – ready to move on to the next type of brainwave, theta waves. 

What are theta waves?

As you drift off into the early stages of sleep, your brain is home to theta waves, which typically have a frequency in the range of 4-7.99hz.

“Theta waves are slower than alpha waves and signify the first stage of non-REM sleep, which is often referred to as light sleep,” Dr Walker says.

“This stage is a period of transition from wakefulness to sleep, and these waves are associated with reduced consciousness. It is during this stage that people often experience sudden muscle contractions, sometimes referred to as ‘hypnic jerks’, and may even have short, vivid dreams.” 

While theta waves may act as a precursor to the deeper stages of sleep, they still offer benefits of their own. It’s during the light sleep stage that our eye movement stops, our heart rate slows and our body temperature decreases, and our brain occasionally experiences unique brainwaves known as ‘sleep spindles’.

These sleep spindles represent short bursts in activity among the slower theta waves, and scientists believe they could play an important role in the formation and consolidation of memory, although more research is needed. 

What are delta waves?

Delta waves (which tend to range between 1-3.99hz) represent the brain’s most relaxed, turned-down state, and occur during the third stage of non-REM sleep – also known as deep sleep.

“This is the most restorative stage of sleep, and delta waves indicate a state of unconsciousness,” Dr Walker explains. “It’s during this stage that the body can repair and regrow tissues, build bone and muscle and strengthen the immune system.”

He continues: “The brain also clears out waste products at a faster rate during deep sleep, potentially helping to prevent diseases like Alzheimer’s.” 

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