Sleep is essential to clear our minds of yesterday’s memories, two new studies have shown.
Although we spend a third of our lives unconscious and paralyzed, scientists have long been baffled by what happens during our hours of shut-eye.
But two research papers published today have offered an explanation.
They explain that at the end of the day, the connections in our brains are tense, saturated with all the conversations, images, and facts we absorbed in our 18 or so hours awake.
Sleep, they say, weakens those connections, essentially wiping the brain clean to leave a blank canvas.
The first of the two new studies is a 13-year-old investigation by Giulio Tononi and Chiara Cirelli at the University of Wisconsin-Madison.
Studying mice, they found that synapses (connections between brain cells) shrunk by around 18 percent during sleep.
These synapses then grew throughout the day, before shrinking once more at night.
To reach this conclusion, Tononi and Cirelli used serial block-face scanning electron microscopy, a state-of-the-art device that can capture snapshots of neurons in two parts of the cerebral cortex.
The researchers made a tiny incision in that brain region, which is responsible for memories, and scanned each layer, day and night.
Then, analyzing their thousands of images, they found key structural changes in the synapses at night, shrinking them, compared to their growth during the day time.
The second paper offers a biochemical explanation for this.
Researchers at John Hopkins University found that when mice fall asleep their synapses absorb a protein called homer1a.
It was this protein that instigated a weakening effect, loosening the connections between synapses that are built up by memories and information.
Conversely, during the day time, homer1a was pushed out of the synapses.
And when the mice were given a stimulant equivalent to coffee, homer1a was prevented from entering the synapses.
The new papers, both published in the journal Science, come months after a Norwegian study used the first brain-imaging of synapses to suggest sleep breaks synapse connections.
To investigate the subject, the team at the University of Freiburg monitored 20 students between 19 and 25 years old, all right-handed non-smokers, free of any mental disorder, drug abuse, or medication use.
In one experiment, the participants had a good night’s sleep (of around seven hours) before being screened.
In the next, they stayed awake for 24 hours – playing games, cooking food, but not drinking caffeine – before undergoing the same test.
To screen them, the researchers zapped magnetic waves at the motor cortex – the brain region that controls movement – to trigger a twitch in the left hand.
They found the sleep-deprived participants barely needed any magnetic pulse to trigger a reaction.
That suggested their brain is more ‘excitable’ than those who are well-rested, and therefore the synpases – which connect brain neurons – are stronger.
Professor Christoph Nissen, the psychiatrist who led the study, then got the groups to carry out word-pair activities that rely on creating new memories.
In these tests, the sleep-deprived participants fared far worse than their well-rested peers.
It suggested that without sleep the brain’s synapses are saturated, tense and strong; filled with all the memories of the day. It makes us tetchy and unable to think clearly.
Just one rough night of sleeplessness, Professor Nissen warned, blocks the brain from resetting, leaving the person foggy, slow and relatively unobservant for the rest of the day.
‘Still we do not really know why we spend such a long time of our lives in this inactive state, so sleep must have a very important function,’ Nissen explained.
‘Otherwise it’s just a very big mistake that evolution made.
‘Our study highlights the importance of sleep, and the notion that sleep is a highly active brain process, not a waste of time.’