Initially, psychologists thought that the brain turns off during sleep and all activity ceases. With the advancements in neuroimaging techniques, which allow us to measure brain activation, we have learned that the brain is, in fact, active when we sleep, and sometimes brain activity during sleep resembles waking brain activity.
The activity observed during REM sleep is comparable to and sometimes greater than waking activity. This surprising discovery gave rise to theories which explain dreaming, like the Activation Synthesis Theory. So, what exactly is REM sleep, and what role does it play?
REM Sleep Meaning
Rapid Eye Movement sleep is quite different to other non-REM stages of sleep. It is characterised by distinct processes occurring within the body and the mind. Let's take a look at what happens during REM sleep and what it tells us about its function.
REM sleep is the fourth stage of the sleep cycle. It is characterised by rapid eye movements, muscle paralysis, increased activation of the brain and the occurrence of vivid and immersive dreams.
REM sleep is particularly interesting because the brain's activity during this stage resembles our waking brain activity more than the activity that occurs during non-REM sleep. That is why it is sometimes called paradoxical sleep.
The brain's electric activity is desynchronised, similarly to what occurs during waking states. In contrast, the brain's electric activity during non-REM sleep tends to be synchronised.
The brain's metabolic activity (the amount of energy it uses) increases to the levels observed during waking activity.
REM Sleep: Dreaming
The Activation Synthesis Theory associates the random activation during REM sleep with dreaming. Further research confirmed that REM states tend to involve vivid and immersive dreams more than other sleep stages.
Williams et al. (1992) compared 60 self-reported dreams and 60 self-reported daytime fantasies from a sample of 12 Harvard students. They found that dreams tend to be more bizarre than daytime fantasies, showed greater plot discontinuities than fantasies and tended to play out in more remote places.
These findings support the idea that dreams arise from random brain activation as they tend to differ significantly from typical daytime fantasies. It demonstrates the differences between REM sleep dreams and fantasies.
The Body During REM sleep
As the brain's activity increases and we experience vivid dreams, our muscles are paralysed to prevent us from acting out our dreams in the real world. Muscle paralysis during sleep is called REM atonia.
Eye muscles are not affected by this paralysis, and we can observe rapid eye movements during this stage. While we are not sure why our eyes move during sleep, one hypothesis suggests that eye movements are the manifestation of us scanning the self-generated visual phenomena when dreaming (Hong et al., 2018).
Heart rate and breathing, which slows down during non-REM sleep, become irregular during REM sleep. We are also partially isolated from outside sensory input. More intense sensations are required to produce arousal during REM sleep.
REM sleep is important not only for brain function but it is also thought to have evolved before the brain did. Cnidarians, like jellyfish, have also been found to sleep even though they don't have a brain.
REM Sleep Cycle
REM sleep is the last and deepest stage of the sleep cycle. It is preceded by three stages of non-REM sleep (stage 1, stage 2 and slow-wave sleep). Let's recap the stages of the sleep cycle.
Stage 1 - The first stage is the shortest; it lasts only 10 minutes and marks the transition between sleep and wakefulness. It is the lightest stage of sleep, during which the brain activity slows down, and the body relaxes.
Stage 2 - During this stage, the body relaxes further, and the brain activity remains slow. K complexes and sleep spindles are characteristic of stage 2 sleep.
Stage 3 (slow-wave sleep) - The heart rate and breathing are at their slowest, muscles are relaxed, and the metabolic activity of the brain drops down, suggesting little brain activity at that time.
REM sleep - Heart rate and breathing become irregular, and muscles are paralysed, eyes move rapidly, brain's metabolic activity is increased. It is characterised by the occurrence of vivid and immersive dreams.
Interestingly, the duration of REM sleep changes throughout the night. With each consecutive sleep cycle, REM sleep gets longer. The first sleep cycle of the night only lasts around 10 minutes, but by the 4th and 5th cycles, it can last for up to 60 minutes.
This means that we get the majority of our REM sleep at the end of the night. By sleeping only 3 or 4 hours a night, you might deprive yourself of most of the REM sleep you could get.
The Role of REM sleep
REM sleep has been associated with memory consolidation, creativity and problem-solving.
To study the role of REM sleep, researchers use nap paradigms, which involve measuring performance on tasks before and after a nap or sleep deprivation paradigms, where the participant is prevented from entering REM sleep for a period of time, from measuring the effects of REM sleep deprivation on task performance.
REM Sleep and Facilitating Learning
REM sleep has been associated with memory consolidation and learning. It has been shown to make up a higher proportion of the sleep cycle during development. Further research has found REM sleep to be particularly important for consolidating non-declarative memory and slow-wave sleep to be important for consolidating declarative memory.
Non-declarative memories refer to memories of skills like drawing and playing instruments, while declarative memories are memories of facts and events.
Mednick et al. (2003) found that participants who experienced REM sleep after learning a non-declarative task performed better than participants who only experienced non-REM sleep. This suggests that REM sleep consolidates non-declarative memories.
REM Sleep and Creativity
Atypical activation patterns during REM sleep and the ability of the brain to produce rich and bizarre dreams during this stage made researchers inclined to believe that REM sleep is important for creative thinking.
Cai and colleagues (2009) compared the effects of REM sleep, non-REM sleep and quiet rest on creativity using a nap paradigm. After REM sleep only, participants performed better on the Remote Associates Test items they had seen before the nap.
These results suggest that REM sleep is important for creating new associations between ideas we were already exposed to.
The Remote Associates Test measures creativity. Participants are shown three seemingly unrelated words (e.g. window, bite, monkey) and asked to think of a fourth word related to the other three.
Figure 1: Bizarre dreams are often reported if a person remembers them after REM sleep.
Side Effects of no REM Sleep
What happens if you are deprived of REM sleep? While we know that REM sleep is important, research on REM sleep deprivation is scarce, possibly because it would be unethical to deprive healthy participants of sleep.
Current evidence shows that short-term REM sleep deprivation is associated with poorer mood regulation, memory consolidation and cognitive flexibility (Kuiken, 2017).
If a person is deprived of REM sleep, they are likely to experience a rebound phenomenon.
The rebound phenomenon occurs when REM sleep is suppressed. After a period of suppression, REM sleep becomes more frequent and intense during the next sleep periods as the body attempts to make up for the lost REM sleep (Nielsen et al., 2005).
What about long-term REM sleep deprivation? Some antidepressant medication is known for reducing REM sleep. However, the daily functioning of people taking these medications for years seems to be unaffected.
Lavie and colleagues (1984) described a case study of a 33-year-old man who experienced almost no REM sleep for the past 13 years. This was a result of a traumatic brain injury that the man experienced when he was 20. After the injury, the man recovered, became a lawyer and appeared to live a normal life.
Thirty-five years later, he was reexamined, and this examination showed he was still experiencing barely any REM sleep. Yet he showed average to high cognitive functioning, normal memory and some difficulties in coordinating movements (Magidov et al., 2018).
Waking Up During REM Sleep
Waking up during REM sleep is characterised by greater arousal than slow-wave sleep. Because the brain is much more active during REM, waking up tends to be easier. Some psychologists even hypothesise that the function of the increased brain activity in REM sleep is to help us wake up once we have got enough slow-wave sleep (Klemm, 2011).
Moreover, we are more likely to remember our dreams when woken up during REM sleep. The dreams reported when woken up during REM sleep tend to be more elaborate, detailed, bizarre and immersive than dreams experienced during the other stages of sleep.
REM Sleep Behaviour Disorder
REM sleep behaviour disorder is a rare sleep disorder linked to neurodegenerative diseases and occurs mostly in the older population. People with the disorder lack sleep atonia; their muscles are not paralysed, which means they can make movements and vocalisations and act out their dreams.
Mike Birbiglia, an American comedian, shared his experience with the disorder. While dreaming about being chased by a missile, he jumped out of a window on the second floor and kept running. Now, to prevent himself from harm during REM sleep, he needs to wear sleep restraints that prevent him from acting out his dreams.
The role of REM sleep - Key takeaways
- REM sleep is the fourth stage of the sleep cycle. It is characterised by rapid eye movements, muscle paralysis, increased activation of the brain and the occurrence of vivid and immersive dreams.
- The brain's activity during this stage resembles our waking brain activity more than the activity that occurs during non-REM sleep. That is why it is sometimes called paradoxical sleep.
- REM sleep has been associated with memory consolidation, creativity and problem-solving.
- Long-term REM sleep deprivation doesn't appear to impact one's cognitive functioning.
- REM sleep behaviour disorder is characterised by a lack of muscle paralysis during REM sleep.
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