What happens when we sleep?

In 1953, some scientists noticed that sometimes, during sleep, people’s eyes make these strange, quick and jerky, side-to-side movements behind closed eyelids. Weird. They discovered in 20 out of 27 people that this jerky eyeball dance, termed “rapid eye movement,” or REM, occurred at the same time as when the people had incredibly visual dreams.

Since this breakthrough, neuroscientists discovered our sleep happens in a cycle of non-REM sleep and REM sleep stages, with most dreaming happening during REM. Amazingly, it was found that some people have vivid colourful dreams, but others only remember their dreams in black and white.

Another strange thing to happen during REM is the temporary paralysis-like state of our arms and legs – it was discovered this is what stops us acting out our dreams! The brain stem takes credit for this; during REM, it sends signals to the muscles that are important for moving our limbs, telling them to relax and have a chill while the brain enters dreamland. Biology knew nobody’s partner would stick around if they got punched every night their girlfriend dreamed of kickboxing.

When I first heard that your eyes move in your sleep, and that some people dream in colour while others recall dreams like old black-and-white movies, I thought it sounded straight out of a sci-fi film. Scientists must have thought this discovery was pretty crazy too because, since 1953, there have been tons of studies about what else goes on when we go to sleep. 

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So, What is Our Brain Up to When We Aren’t Conscious Enough to Notice?

One of the major reasons why we need to sleep is because sleep helps us to keep, stabilise, or consolidate our memories. If we don’t consolidate them, our memories become very difficult or impossible to recall in the future. 

When we are awake, as we perceive events or comprehend information, signals are passed between neurons in our brain, forming a sort of connection pathway. 

When we sleep, these initially formed, kind of unstable connections are retraced, reprocessed, and consolidated as a memory, which is then integrated into our bank of long-term memories that already exist. 

Scientists believe this happens during sleep because this way the neuronal connections that occur as a person experiences stimuli, and the pathways traced, or reprocessed when consolidating memories the person has just formed, won’t interfere or disrupt each other. This also avoids the brain forming confusing memories or hallucinating!

The process of memory consolidation in our brains during sleep still poses a lot of debate in the neuroscientific and psychological community, making it still a hot topic for research.

There Are Different Ways We Sleep

Human sleep cycles between slow-wave, deep sleep (SWS), and REM sleep. Many questions about how and when memories are consolidated are still unanswered, but it is thought it occurs during both SWS and REM sleep. Slow-wave sleep happens to us between 1-4 stages and is followed by REM sleep. 

The Sleep Stages

Stage 1 non-REM is where our heart rate and breathing slows down, and we fall from being awake into a light slumber. If you ever have had a sudden twitch just before you fall asleep, this is when it happens, as our muscles relax. During the few minutes that stage 1 lasts, our brain waves start to slow way down. 

Stage 2 non-REM is the stage we spend most of our sleeping cycle in each night. During this phase, our heart rate, breathing rate, and body temperature decreases, and our brain waves are super slow with little electrical bursts of activity. Oddly, our eye movements stop in this stage.

We experience the deep sleep, of really slow brain waves that humans need to feel well-rested, during stage 3 and 4 of non-REM. At this time our heart and breathing rate is the slowest of the sleep states, and it can be kind of hard to wake up. 

REM (The Weirdest Sleep Stage)

REM is definitely the strangest part of being asleep. Happening around 90 minutes after we fall asleep, during REM, our eyes move, our heart rate and blood pressure increase, and our breathing is more irregular and faster.

REM sleep is thought to be really important in processing our emotional memories, with people shown able to remember emotional memories more clearly after a period of REM-rich sleep – mad! It has also been shown that reduced, less-than-normal periods of REM sleep are reported in negative emotional states like major depression, grief, and more commonly, PTSD.

Interestingly, during REM sleep, the thalamus is active.  The thalamus is the part of the brain that passes information from the senses, like sound or touch, to the brain’s covering called the cerebral cortex. The cerebral cortex interprets and then passes this sensory information from the short-term to the long-term memory.

Normally, in most sleep stages, the thalamus is pretty asleep itself, allowing us to slumber peacefully, ignoring the world and everything that normally tingles our senses. But, the thalamus has a great time during REM; being active and functioning during REM this little part of the brain sends the cortex sounds, colours, images, and all kinds of other sensations to fill our dreams.

This may be why sometimes things you hear from the external world, such as a doorbell, when you’re catching some z’s, can become incorporated into your dreams about falling from an airplane, or winning the lottery.


Why Do We Need Sleep?

A lot of research has shown that sleep helps us store our emotional memories, with one study even showing that the effects a 3-hour-long nap after learning something had on preserving emotional memory could be detected 4 years later! Fascinatingly, some scientists even suggested that one way to combat post-traumatic stress disorder was to deprive a person of sleep immediately after their trauma – so that they would retain less of the negative emotional memory. 

Sleep, our unsung hero, helps us develop problem-solving skills, process our emotions, learn new things more easily when we are awake, and reduces the likelihood of psychiatric disorders. This mysterious phenomenon, our human stand-by state, continues to baffle and surprise researchers all over the world even after centuries of studying the z’s. 

Emma Monaghan
Emma Monaghan

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