What happens in the brain when we sleep?

What occurs in the brain when we are deep in slumber? What are the different stages of sleep and what role do they play in learning and memory formation? What about in anxiety and pain? Do neurons and neurotransmitters also play a role? These are the questions we will tackle in this Special Feature, using the latest evidence available. We round up the neuroscientific evidence that helps explain the intricate workings of the human brain when it is asleep. Scientists generally agree that there are four stages of sleep that we cycle through several times each night. The first three form the so-called non-rapid eye movement (REM) sleep and the fourth one is REM sleep — where dreams occur.

In the first non-REM stage, the body and brain transition from wakefulness to sleep. The brain changes its electrical oscillations from the active, wakefulness pattern of brainwaves into a slower rhythm.

Muscle tone throughout the body relaxes. This is the phase during which our bodies may twitch as we enter slumber.

The second non-REM stage involves a drop in the body’s temperature, the heartbeat and breathing become slower, and the brainwaves slow down further. Short bursts of electrical activity in the brain may still characterize this stage of sleep.

The third stage of non-REM sleep is the deep sleep stage, which our bodies need to wake up feeling refreshed and restored. In this stage, heart rate, breathing, and brain activity all drop to their lowest point.

The REM, dream-filled light-sleep stage is the fourth and last one. According to the National Institute of Neurological Disorders and Stroke (NINDS) , REM occurs about 90 mins after falling asleep.

REM sleeps lasts roughly 10 minutes the first time, increasing with each REM cycle. Rapid eye movement is so-called because the eyes quite literally move rapidly behind closed eyelids.

During REM, breathing becomes more rapid and irregular, heart rate and blood pressure increase to near waking levels. An interesting fact about REM sleep is that people experience less and less of it as they grow older.

One of the two main things that control sleep is the ensemble of “physical, mental, and behavioral changes that follow a daily cycle” — called circadian rhythms . The term “circadian” comes from the Latin circa , meaning “around” and dies , meaning “day.”

Circadian rhythms respond to the light-darkness cycle and are genetically predetermined, at least in part, and dictated by so-called biological clocks — proteins that interact within cells in every tissue and organ in the human body.

The suprachiasmatic nucleus , a structure in the brain formed by a group of about 20,000 neurons , or nerve cells, coordinates all the biological clocks.

Secondly, the sleep-wake homeostasis also tracks a person’s need for sleep and dictates when they get sleepy. The so-called homeostatic sleep drive increases with the time that a person spends being awake. Its visible effects on brain activity and connectivity between neurons have been well documented .

Another area that has been the focus of much research is the relationship between sleep and learning or memory formation. Scientists know for sure that sleep is crucial for learning — but which stage of sleep is more important?

Does learning occur in the light REM sleep stage or the deep, non-REM phase of sleep? How do neurons in different brain areas coordinate across sleep stages to facilitate learning and memory consolidation?

Two studies that Medical News Today reported on help to shed light on these questions. Sleep helps the brain learn and stay flexible

In the first study , the experimenters tampered with the study participants’ deep, non-REM sleep stage after asking them to learn a new set of movements. The scientists monitored the participants’ brain activity — their motor cortex, specifically — throughout the study.

The team — led by Switzerland-based scientists — found that a restless deep sleep resulted in a visibly reduced learning efficiency. The researchers’ explained that their results hinged on the brain’s synapses and their roles in learning.

Synapses are microscopic connections between neurons that, together with brain chemicals, or neurotransmitters, facilitate the passing of electrical impulses from one neuron to another. During the day, synapses switch on in response to the stimuli that the brain receives from the environment.

But during sleep, the activity of these synapses goes back to normal. Without this restorative period, they stay excited at their peak activity for too long.

This interferes with the brain’s neuroplasticity — that is, its ability to re-wire itself and create new connections between neurons. Neuroplasticity enables the brain to ‘pick up’ new skills, change and adapt to its environment stimuli, and ultimately learn new things.

Nicole Wenderoth, a professor in the Department of Health Sciences and Technology at the ETH Zurich, and co-lead author explains what occurred in their new study. “In the strongly excited region of the brain, learning efficiency was saturated and could no longer be changed, which inhibited the learning of motor skills.” To the authors’ knowledge, this was the first study that showed a causal relationship between the deep phase of sleep and learning efficiency. “We have developed a method that lets us reduce the sleep depth in a certain part of the brain and therefore prove the causal connection between deep sleep and learning efficiency,” says study co-author Prof. Reto Huber. Sleep also helps us unlearn

The second study that MNT reported on looked at different sleep stages. However, this research showed that sleep does not just enable the brain to learn new things but also unlearn.

The original 2017 study involved an auditory learning task. The researchers played sound sequences while the participants were asleep and awake.

They monitored the volunteers’ brain electrical activity using an electroencephalogram (EEG).

The EEGs also captured sleep spindles that occurred when the sleeping brain learned new sounds. Sleep spindles are spikes in oscillatory brain activity that previous research has linked with learning and memory consolidation.

After each sleep session, the experimenters asked the participants to re-listen to the sound sequences and recognize them. They assessed their learning performance through tests.Using the EEG readings, the […]

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