We all know that if we want what we've studied during the day to stick, it's best to get a good night's sleep. And while scientists have long understood that our memories rely on connections being built between neurons in our brains, it's not been clear how sleep actually helps to consolidate that information.
Now, two new studies have found biological evidence that expains the age-old wisdom that if we want to remember, we need to sleep to forget.
It's natural that we're curious about why we fall unconscious for up to 16 hours every day. One recent hypothesis suggested that sleep flushes out potentially toxic proteins which build up in the brain during the day.
And recent studies have found that if we don't get enough sleep, we increase our risk of developing cardiovascular disease and type 2 diabetes, not to mention neurodegenerative disorders such as Parkinson's disease.
Now, a pair of studies from the University of Wisconsin and Johns Hopkins University have provided evidence for another benefit to sleeping, suggesting that it allows us to 'prune' our memories and fine-tune the lessons we've learned while awake.
The so-called synaptic homeostasis hypothesis isn't new - researchers from the University of Wisconsin developed the idea over a decade ago, proposing that sleep allows our brains to cut back the connections we develop between our neurons while we're awake in order to make our memories clearer.
As we experience new things, our brains build or weaken connections called synapses that link our nerve cells. The behaviours and memories we develop are encoded in these interconnected webs of neurons, which rely on the size and strength of the synapse to communicate messages effectively.
The biologists responsible for the synaptic homeostasis hypothesis argue that the process of building such networks is a little too enthusiastic during our waking hours. Sleep provides a quiet opportunity for the brain to selectively downscale the networks we build while we're conscious.
There's plenty of evidence that neurons can cut back synapses, in some cases through cells that patrol the brain looking for synapses to swallow up. Comparisons of synapses following periods of sleep and sleep deprival might also be indirect evidence of the brain reshaping synapses as you snooze.
The team from the University of Wisconsin has now added more even evidence to back up their hypothesis.
Over the past four years, the biologists analysed thin brain shavings from a handful of mice which had slept, another few which had been kept awake and entertained with toys, and a few more which were kept awake but unstimulated.
The scientists then measured the size and shape of some 6,920 synapses across hundreds of brain sections. Slices taken from the sleeping mice contained synapses which were 18 percent smaller than the synapses in those which were awake.
Tellingly, those synapses with a reduced connection were significantly limited to nerves with smaller dendritic spines – protrusions which take information from a synapse to the nerve's axon. In other words, established learning was being spared the chop.
In 2014, team member Giulio Tononi of the University of Wisconsin's Centre for Sleep and Consciousness explained,
"During wake, learning strengthens the synaptic connections throughout the brain, increasing the need for energy and saturating the brain with new information. Sleep allows the brain to reset, helping integrate, newly learned material with consolidated memories, so the brain can begin anew the next day."
A second team of biologists from Johns Hopkins University attacked the question from a different angle.
They tagged proteins on the synapses of living mice with a fluorescent marker, and then literally watched the mouse brains as they slept. As one would expect if the synapses were shrinking in size, the markers decreased. Further analysis revealed a 20 percent drop in the receptor AMPA.
Next, the researchers genetically engineered mice without a type of protein called Homer1A, a chemical known to trigger the removal of receptors in synapses. These mice slept just like their non-engineered cousins.
But unlike the Homer1A-intact mice, their synapses held onto their receptors.
In order to study the effects this would have on memory, mice were run through a box with an electrified floor panel. That night, some of the mice were treated with a chemical which prevented Homer1A from entering the dendritic spines.
When run through the same box the next day, all of the mice froze in fear. Placed into a different box, however, those mice with reduced synapses bravely checked them out. Mice which had been treated with Homer1A blockers, however, froze up again.
Though hard to tell what's going through their tiny rodent minds, the researchers concluded these mice were confused by similarities between the boxes. Rather than having a sharp recall of the previous day's experience, the mice found it hard to distinguish what was important.
Sleep is an incredibly complex behaviour that's associated with a whole bunch of biological functions, from our immunity to our digestion. So it's hard to point the finger at any one 'reason' for why we sleep.
But given the impact of modern technology on our sleep cycles, it's important that we pay attention to the multitude of benefits of getting a full night's snooze.