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Lifebrain Monthly E-newsletter August 2019 

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The sleeping brain

Humans spend about one third of our lives asleep. This means that across a 72-year lifespan, which is the worldwide average life expectancy, a total of 24.4 years will be spent in bed (assuming 8 hours of sleep per night). Modern sleep research began less than a century ago, when Daniel Kleitman, known as the “father of sleep research,” founded the first sleep laboratory in 1925. Since then, researchers have uncovered many of the mysteries of the nature and function of sleep.

Source: Colourbox

What is sleep?

Human sleep is characterized by the cyclic occurrence of two main types of sleep: non-rapid eye movement (non-REM) and rapid eye movement (REM) sleep, which are associated with specific patterns of brain- and muscle activity. Non-REM sleep can be further divided into stage 1, 2 and 3, each again associated with unique activity patterns.

Non-REM sleep:

Stage 1
Stage 1 marks the transition from wake to sleep, we start to feel drowsy, drifting in and out of sleep. The eyes starts to roll slowly, and muscle activity is relatively low. During this stage we are easily awakened, and if we do wake up, we may even deny having slept at all.

Stage 2
Stage 2 is an intermediate stage, with eye movements, muscles and brain waves continue to slow down.

Stage 3
Whereas stage 1 and 2 are defined as light sleep stages, stage 3 is deep sleep, where the slowest brain waves dominate. Stage 3 is also called slow wave sleep (SWS). There is even less muscle activity, and very little eye movement during stage 3. This is the most refreshing and restorative of all the sleep stages, and the one that it is most difficult to wake us up from.

REM sleep:

REM sleep is the final sleep stage. The eyes move rapidly from side to side in a characteristic manner, giving name to this stage (“rapid eye movement”). While the brain is highly active, much like when we are awake, the muscles of the body are relaxed. It is during REM sleep that most dreams occur, and the muscle relaxation prevents us from acting out our dreams.

We cycle through all stages of non-REM and REM sleep several times throughout the night (See illustration below).
Source: Illustration of the author

One sleep cycle usually lasts about 90-100 minutes. During the first half of the night we spend more time in deep sleep, whereas the night progresses, time spent in deep sleep decreases and the periods of REM sleep get increasingly longer. You may wonder, how can we can know all this? How is sleep monitored?

Monitoring of sleep

Monitoring of sleep is based on recordings of:

1) electrical activity in the brain

The electrical activity in the brain differs from wake to sleep as well as between the various sleep stages. By attaching electrodes to the head, this activity can be recorded. Recording of brain wave activity is called electroencephalo-graphy (EEG). 
EEG records brain waves (Source: Colourbox)


2) tension of muscles in the body

Muscle activity is recorded using electromyography (EMG), as muscle tones also differ between wake and sleep, and between sleep stages. 


3) eye movements

Eye movements are also recorded using electrooculography (EOG; of the small eye-moving muscles), mainly to identify rapid eye movement.

The combination of EEG, EMG and EOG is called a polysomnography (PSG), referring to the multiple (‘poly’) signals needed to monitor sleep.
Electrooculography records eye movements (Source: Colourbox)

Why is sleep important?

One might think that the brain is “shut down” when we sleep and that sleep is waste of time. However, the lifelong internal drive to sleep, and its apparent universality across animal species, suggest that sleep is vital. We know that our brains remain highly active when we sleep. Lack of sleep have many negative effects, both on our health and mental functions.
Sleep is necessary for having a fully functioning brain when we are awake; perhaps because sleep helps to restore the energy consumed by the body and brain throughout the day.
Sleep may also enable removal of harmful material that accumulate in the brain when we are awake. Another possibility is that sleep plays a crucial role in learning and memory, especially by aiding long-term storage of memories. We will write more about this in our next newsletter!

The referred studies

Malhotra, R.-Avidan, A.: Sleep stages and scoring technique
Rasch, B.-  About Sleep's Role in Memory. Physiol Rev. 2013 Apr; 93(2): 681–766.
doi: 10.1152/physrev.00032.2012

Xie et. al.: Sleep Drives Metabolite Clearance from the Adult Brain. Science. 2013 Oct 18; 342(6156): 10.1126/science.1241224. doi: 10.1126/science.1241224

Source of newsletter

This newsletter was edited by Hedda Ness, PhD student, Centre for Lifespan Changes in Brain and Cognition, Department of Psychology, University of Oslo.


Your comments are always valuable to us, so do not hesitate to contact us.

Center for Lifespan Changes in Brain and Cognition at the University of Oslo
Kristine B. Walhovd project coordinator
Barbara B. Friedman administrative coordinator
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This project has received funding from the European Union ’s Horizon 2020 research and innovation programme under grant agreement No 732592.
Copyright © 2019 Lifebrain Horizon2020 project, All rights reserved.

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