Sleeping – a restful, passive time – but not for the brain

This post is also available in Dutch.

Although sleep appears to be a restful and passive time for the mind, the brain is everything but inactive. During sleep our brain activity is highly organized with different activation patterns separated into different sleep states. So what does the brain do during sleep?


Image by rachel CALAMUSA – It’s time to sleepUploaded by xiaphias, CC BY-SA 2.0,

Sleep is essential to our health, cognition, concentration, memory and mood. After decennia of research, the mysteries of sleep have still not fully been revealed. However more research points to its role in clearing waste from the brain, refueling essential neurotransmitters to support learning and memory.

How do we investigate sleep in the laboratory?

There are different ways to investigate the brain mechanisms of sleep, but one of the main research methods used is electroencephalography (EEG). This method consists of a cap attached with electrodes that measure the electrical activity of brain cells transmitted to the surface of one’s head. It is an easy and non-invasive method with which “brain waves” or brain activity patterns can be recorded with the temporal resolution of milliseconds. EEG is especially useful in sleep research as sleep has characteristic repetitive activation patterns that cycle during sleep.

Brain cycles

Normal sleep can be separated in two states – rapid eye movement (REM) – and non-rapid eye movement (nREM), alternating during sleep. The nRem sleep is characterized by synchronous brain activity and appears to have a ‘regular’ pattern of activity. It is also associated with low muscle tone and reduced psychological activity. REM sleep, in contrast, shows characteristic desynchronized brain activity and appears to be a bit more ‘chaotic’, similar to waking states (see the brain activity related to awake, nREM and REM on an EEG here). REM sleep is associated with bursts of rapid eye movements, vivid dreaming, activation of the cortex and the paralysis of the skeletal muscles; in order to probably prevent us from acting out of our dreams. Sleep normally begins with nREM sleep which progresses through deeper nREM sleep stages before it switches over into REM sleep. This switch happens after approximately 80 to 100 minutes after sleep onset. The nREM sleep and REM sleep cycle for a duration of about 90 minutes.

A small brain region dictates nREM or REM sleep

Opposing the view that various brain regions affect REM sleep, a year ago researchers were able to show that there might be one key region dictating whether we switch into REM sleep or not. Neuroscientist activated neurons in the medulla, one of the oldest evolved regions of the brainstem, of sleeping mice. With this, they were able to switch the sleep state of mice from nREM sleep into REM sleep. This was also true the other way around, as REM sleep was reduced or even prevented by the deactivation of neurons in the medulla (read more about the study here).

More information

Russell Foster on why do we sleep?

A link on how REM sleep controls nREM sleep.


Written by Mahur.

Edited by Roselyne.


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