By Peter M. Vishton, PhD, William & Mary

Edited by Kate Findley and proofread by Angela Shoemaker, The Great Courses Daily

You may think of sleep as one long stretch of blank space, but your brain is actually quite active. Professor Vishton breaks down the different stages of sleep and what your brain is doing in each.

Measuring Sleep with the EEG

When it comes to charting the different stages of sleep, the most important tool in the sleep researcher’s kit is the electroencephalograph, commonly called an EEG. It records tiny electrical signals produced by neurons in the brain. 

Many circuits in the brain produce variations in the amplitude of brain waves, each of which takes the shape of a sine function. The voltage cycles up, then back down, and then back up. 

Some brain circuits oscillate quickly, producing high-frequency waves. Other circuits cycle slowly, producing low-frequency waves. 

When the EEG records the electrical activity, it captures a sum of all these waves put together. The raw data from an EEG doesn’t look wavelike at all because all these different frequencies are essentially piled up on top of one another.

The first thing that a sleep scientist will do with this EEG data is to apply a mathematical process called a Fourier transform. This process takes a complex mixture of different frequencies and decomposes that mixture back into the underlying sine functions. 

The outcome of this Fourier transform is something called a power function. We can look at that decomposition and observe how much energy is present in the brain at different frequencies.

The Beta and Alpha Sleep Stages

When you’re awake, your power function would have a lot of energy in the range of 13–24 cycles per second. This energy pattern is associated with normal, waking thought. 

Sleep researchers refer to the energy in this particular frequency as beta waves. The power spectrum of beta waves indicates an active, wakeful brain.

As you relax at the end of the day and get ready to fall asleep, the energy in that beta range drops, while the energy in the alpha range of 8–12 cycles per second increases. The beta waves become less pronounced, and the 8–12 cycle alpha waves take their place. 

You don’t need to be asleep to make this happen, though. If you meditate, relaxing and clearing your mind, you’ll shift a lot of your brain activity from beta waves to alpha waves.

When you become relaxed and start to drift off to sleep, the energy in this alpha range drops off and is replaced by theta waves in the range of 4–7 cycles per second. As this happens, you lose consciousness. You are lightly asleep.

Moving through the Cycles

Sleep researchers call this stage 1 sleep. You and your brain will typically remain in this state for 10–30 minutes before shifting into stage 2 sleep, then 3, and then 4. 

As you pass into each of these stages, your brain continues to produce more and more energy in slower and slower frequency ranges. In the deepest stages of sleep, your brain will be producing a great deal of delta wave activity, at a frequency of less than four cycles per second.

This progression into deeper and deeper sleep sounds like an engine revving down to stop, as if the brain is slowing down and halting activity. The energy output of the brain, however, doesn’t drastically change. 

The waves of activity that it produces are slower than when you’re awake, but the deeper the sleep, the higher the amplitude. In the deepest sleep, your brain waves have a much higher amplitude. 

There are fewer waves, but they’re bigger. Just to be clear, your brain isn’t turning off at all when you go to sleep; it’s just shifting into a different mode of activity. There are more and more delta waves, in place of beta, alpha, or theta waves.

Your brain and body step down through four stages of deepening sleep, from awake to very deep sleep. Essentially everyone exhibits this pattern of activity as they sleep for the first hour or so. 

Entering Deep Sleep

You remain in deep, delta wave sleep for about a half hour or so after. If you ever try to wake someone when they’re in this fourth, deepest stage of sleep, you’ll find that it’s a hard thing to do. It can take several minutes, during which time the sleeper often seems confused or disoriented.

Presuming that no one wakes you up, you’ll spend a half hour or so in this deep sleep. After this half hour, your brain waves begin stepping up their rate of activity, back toward the place you were at the beginning, producing fewer delta waves and more theta waves, then more beta waves. 

At this point, you will have stepped back up to stage 1 sleep. At the top of these steps, however, you don’t continue and wake up. A really remarkable set of changes emerges. 

The activity level of the brain becomes very much like that of someone who is awake. Lots of energy shows up in the alpha range. This is called REM (rapid eye movement) sleep, which is the stage you need to spend a good amount of time in to wake up feeling refreshed.

This article was edited by Kate Findley, Writer for The Great Courses Daily, and proofread by Angela Shoemaker, Proofreader and Copy Editor for The Great Courses Daily.