Sleep is essential because it both helps our brain do a regular “deep-clean”, while not getting enough can lead to neurological disorders like Alzheimer’s disease.
This is the caution given by two sets of researchers this week who investigate the role that a good night’s rest gives us — and what happens when we dream.
When we “slip into the arms of Morpheus” each night, we should be getting around seven to eight hours’ worth of slumber, one or two of which counts as deep sleep.
For the remaining time, we experience so-called REM sleep, named for the “rapid eye movements” associated with it, in cycles throughout the night.
This is the time — particularly in the last hour of sleep, close to waking, which is nearly all REM sleep — in which we have those dreams we are more likely to remember upon waking.
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Professor Drew Dawson and Dr Madeline Sprajcer are both specialists in the science of sleep and fatigue.
Writing in the Conversation, they said of sleep: “Anything we spend so much time doing probably serves multiple ends.
“Dreams and nightmares are mysterious and we’re still learning about them.
“They keep our brains ticking over. They wash the thoughts from the day’s events at a molecular level. They might even help us imagine what’s possible during our waking hours.”
Some experts think we might dream in order to stop our bodies from getting too cold — with the core temperature typically higher at these times — or to keep us from sleeping too deeply and become vulnerable to attack.
The experts explain: “It is typically easier to wake from dreaming if we need to respond to external cues or dangers.
“The brain activity in REM sleep kicks our brain into gear for a bit. It’s like a periscope into a more conscious state, observing what’s going on at the surface, then going back down if all is well.”
All mammals, the duo note, are thought to dream — even weird egg-laying ones like echidna and platypuses — and experience sleep in stages and with brain activity similar to humans.
In contrast, less evolved species do not sleep, with the odd exception.
The pair explain: “Some jellyfish — who do not have a brain — do experience what could physiologically be characterized as sleep.
“But they do not experience the same physiological and behavioral elements that resemble REM dream sleep.”
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Dawson and Sprajcer continue: “REM sleep is important for ensuring our brain is working as it should, as indicated by studies using electroencephalography, which measures brain activity.
“In the same way deep sleep helps the body restore its physical capacity, dream sleep ‘back-flushes’ our neural circuits.
“At the molecular level, the chemicals that underpin our thinking are bent out of shape by the day’s cognitive activity.
“Deep sleep is when those chemicals are returned to their unused shape. The brain is ‘washed’ with cerebrospinal fluid, controlled by the glymphatic system.”
At the same time, the duo explain, dreams also help the brain “tidy up” our recent memories and feelings from the day, and discard the information it doesn’t want to keep — this is why we often dream about things that happen during the day.
REM sleep is when our brains consolidate emotions and “procedural memories” of how to do tasks, and the rest being when we file “episodic memories” — that is, events.
But, you may well ask, what about nightmares like that one with the man-eating tiger, or where I didn’t study for my history exam?
Dawson and Sprajcer have the answer: “As our night’s sleep progresses, we produce more cortisol — the stress hormone.
“It is thought the amount of cortisol present can impact the type of memories we are consolidating and potentially the types of dreams we have.
“This means the dreams we have later in the night may be more fragmented or bizarre.
“Our daytime experiences can also fuel nightmares or anxiety-filled dreams after a traumatic event.”
Because sleep plays such a vital physiological function, it will come as no surprise that not getting enough sleep is not good for the body or the brain.
While a poor night’s sleep might make us feel a little off, past research has shown that sleep loss over long periods can cause damage to the hippocampus — the part of the brain involved in both learning and memory — increase your risk of Alzheimer’s and other neurological diseases.
In a recent study, Professor Jia Mi of the Binzhou Medical University in Shandong, China and colleagues set out to explore how exactly it is that sleep deprivation can cause such harm, using mice with induced insomnia.
They identified a protective protein whose level decreases when we don’t get enough shuteye — resulting in the death of the neurons in our brains.
The team began their experiments by assessing how well mice could navigate a simple maze and learn to recognize new objects in the wake of two days of sleep deprivation.
They then extracted proteins from within the animals’ hippocampi to determine which had changed relative to the baseline.
To narrow down their focus, the team cross-referenced these proteins with information from previous experiments into maze performance in mice that were not sleep deprived.
This led them to pleiotrophin, a protein which was found to decline in the sleep-deprived rodents.
Further investigation revealed that a molecular pathway by which a loss of pleiotrophin causes cells within the hippocampus to die — showing how sleep protects brain function.
Pleiotrophin, the team noted, has also implicated in Alzheimer’s and other neurodegenerative diseases.
Mi and colleagues also suggest that pleiotrophin levels might be used to serve as an indicator of insomnia-related cognitive impairment.
The full findings of the study were published in the journal Journal of Proteome Research.
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