The purpose of sleep appears to be to help forget some memories so others can be stored for later use, two major new studies suggest.
For years scientists have struggled to work out exactly why humans need to sleep for about a third of every day.
The new research adds fresh evidence to the idea that it is a key part of the way the brain files information.
And one of the academics suggested that while sleeping pills might help people doze off, they could interfere with this process.
In one study, mice were put in an unfamiliar arena and given a mild electric shock. Some were then given a drug to prevent the brain from re-ordering its memories, while a second group had a normal night’s sleep.
When the mice were returned to the same arena, they remembered the shock and spent much of the time motionless.
When they were put into another unfamiliar arena, the well-rested mice were slightly hesitant but explored it as a normal mouse might.
However the drugged mice froze more often as if the fear of the shock remained even though they were in a different place, the researchers reported in the journal Science.
It is thought that this happened because the drug interfered with a process known as “scaling down”, which is thought to be key to forgetting some memories and filing others away.
One of the researchers, Dr Graham Diering, said: “We think that the memory of the shock was stronger in the drugged mice because their synapses couldn’t undergo scaling down, but all kinds of other memories also remained strong, so the mice were confused and couldn't easily distinguish the two arenas.
“This demonstrates why ‘sleeping on it’ can actually clarify your ideas.
“The bottom line is that sleep is not really downtime for the brain. It has important work to do then, and we in the developed world are short-changing ourselves by skimping on it.”
Dr Diering, of Johns Hopkins University, said it was currently thought that information is actually “contained” within synapses, the connections between brain cells.
“Our findings solidly advance the idea that the mouse and presumably the human brain can only store so much information before it needs to recalibrate,” he said.
“Without sleep and the recalibration that goes on during sleep, memories are in danger of being lost.”
In the second study, researchers from the Wisconsin Centre for Sleep and Consciousness used an electron microscope to take pictures of what happened to the synapses of mice while they were awake and asleep.
They found concrete evidence for this “scaling” process – the synapses grew strong and large when the mice were active and then shrank by about 20 per cent during sleep.
It is thought this effectively creates more room for further growth in memories the following day. However 20 per cent of the synapses did not change size, and it is thought that these hold the most stable memories.
One of the researchers, Dr Chiara Cirelli, said: “This shows, in unequivocal ultra-structural terms, that the balance of synaptic size and strength is upset by wake and restored by sleep.
“It is remarkable that the vast majority of synapses in the cortex undergo such a large change in size over just a few hours of wake and sleep.”
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