Brain During Sleep

There are many theories surrounding sleep and dreaming, from Freud's Theory of Dreaming and the famous Freud Wolfman Study to Hobson and McCarley's (1977) Activation Synthesis Theory of Dreaming. But what does our brain do while we sleep and dream? Typically, examining brain waves whilst the brain is sleeping reveals a lot about what our brain is up to while we slumber. We can see brain waves using an electroencephalogram (EEG).

Let's explore the function and actions of the brain during sleep.

• First, we will explore the brain's activity during sleep.
• Then we will look at brain wave activity during sleep.
• After, we will examine brain activity during deep sleep.
• Finally, we will identify what brain cleansing during sleep is.

Fig. 1 - The brain is quite active during sleep.

Brain Activity During Sleep

The brain is still very active while we are sleeping. There are four stages of sleep (which you will go through roughly five times during the night - called the sleep cycle) with rapid-eye movement (REM) sleep being the fifth part of the cycle overall.

The 4 stages you will cycle through when asleep are:

1. Stage One: Light sleep that someone can easily be woken from, muscles relax and eye movements start to slow.
2. Stage Two: Occurs later in the night when falling deeper into sleep. Your body temperature cools, your heart rate becomes slower and your eye movements stop. There are bursts of brain activity called spindles. K-complexes also occur.
3. Stage Three: The stage between light and deep sleep, where brain activity starts to slow.
4. Stage Four: Deep sleep, when all brain activity is slow with non-rapid-eye movements and it is difficult to wake someone.

REM sleep is when the brain is most active during the sleep cycle, often occurring around 90 minutes into first falling asleep, and occurs periodically throughout the different stages of the cycle, lasting slightly longer after the last cycle until waking. REM sleep is where dreaming takes place, typically people spend roughly two hours a night dreaming.

A sensory blockade occurs during REM sleep, meaning that information from the five senses (sound, sight, smell, taste and touch) is not interpreted by the brain. Signals (brain waves) for REM sleep start at the base of the brain from an area in the brainstem called the pons, which prevents movement by shutting off neurons in the spinal cord and is known as movement inhibition.

According to the activation-synthesis theory of dreaming, dreaming occurs when these signals from the pons reach higher brain areas such as the cerebral cortex (which interprets sensory information when awake), resulting in a synthesis of information using memories to piece together bits of information. The limbic system is also activated by these brain waves during sleep, which is an area of the brain involved in managing emotions.

According to the theory, activation of the limbic system is why dreams can often feel particularly emotive or powerful.

Brain Wave Activity During Sleep

Brain waves vary in number and length during different stages of the sleep cycle. They reveal the general type of activity occurring during sleep, and have led to interesting theories regarding what it is exactly the brain is up to during sleep.

Different brain waves occur during different stages of a sleep cycle. Whilst awake, we typically see Beta waves (high frequency and relatively low amplitude). Brain waves are measured using an electroencephalograph (EEG). This device uses electrodes (used to conduct electricity that can measure the electric impulse of the brainwaves), that are placed on a person's scalp.

Brain waves are measured by frequency, recorded in hertz (Hz), and amplitude.

1. Stage One: Alpha and Theta brain waves dominate the first stages of sleep. Alpha waves are more synchronised than Beta waves and indicate a relaxed, restful state. They are lower in frequency and higher in amplitude. Theta waves are the next step in stage one, hence why stage one is seen as a transitional stage between wakefulness and sleeping, with an even lower frequency and higher amplitude than Alpha waves.
2. Stage Two: Theta waves are present, a deeper stage although not as deep as Stage Three or Four of the sleep cycle. Here, we can usually see spindles (bursts of high-frequency activity) and K-complexes (high amplitude activity).
3. Stage Three: A transition to deep sleep, Stage Three is dominated by Delta waves, and is a transition to Stage Four.
4. Stage Four: Delta waves are primarily seen in deep sleep, and have the lowest frequency and highest amplitude of all brain wave activity.
5. REM Sleep: Interestingly, REM sleep brain waves resemble the brain waves of a person who is awake.

Fig. 2 - Brain waves can be categorised.

Notice how brainwave lengths get faster down the list from delta through to gamma? That's what activity happens in the brain when waking up, beta brainwaves occur when concentrating and gamma brainwaves occur during perception and consciousness (so when awake).

Brain Activity During Deep Sleep

Slow delta waves are the most common brainwaves seen during deep sleep, but this does not mean that the brain is inactive during this stage of sleep. In fact, the main functions of deep sleep are vital for our health and well-being when awake.

On top of restoring energy and creating feelings of restfulness, deep sleep is an important factor in learning and development. Deep sleep supports the facilitation of short-term and long-term memories, which include memories used for processing new information involved in learning.

Growth is also stimulated during moments of deep sleep as hormones are secreted by the pituitary gland, which also aids in the repair of bones and tissues after a long day or injury. Hormones (e.g., oestrogen) secreted in deep sleep can

aid in cell regeneration and building a stronger immune system.

Brain Cleansing During Sleep

Relatively new recent findings suggest that deep sleep may play an important role in 'cleansing' toxins from the brain. Using neuroimaging techniques to measure brainwaves and physiological changes in the brain, researchers discovered a slight pattern of cerebrospinal fluid (CSF) changes that appear alongside delta brainwaves during non-rapid eye movement in deep sleep (Fultz et al., 2019).

CSF is suggested to remove metabolic waste products while moving in and out of the brain. The results of the study suggest that waste is removed during periods of non-REM deep sleep.