Every animal sleeps, but the reason for this has remained foggy. When lab rats are not allowed to sleep, they die within a month.
One idea is that sleep helps us strengthen new memories. We know that, while awake, fresh memories are recorded by reinforcing (加强) connections between brain cells, but the memory processes that take place while we sleep have been unclear.
Support is growing for a theory that sleep evolved so that connections between neurons(神经元) in the brain can be weakened overnight, making room for fresh memories to form the next day.
Now we have the most direct evidence yet that he is right. The synapses in the mice taken at the end of a period of sleep were 18 per cent smaller than those taken before sleep, showing that the connections between neurons weaken while sleeping.
If Tononi's theory is right, it would explain why, when we miss a night's, we find it harder the next day to concentrate and learn new information — our brains may have smaller room for new experiences.
Their research also suggests how we may build lasting memories over time even though the synapses become thinner. The team discovered that some synapses seem to be protected and stayed the same size. "You keep what matters," Tononi says.
A. We should also try to sleep well the night before.
B. It's as if the brain is preserving its most important memories.
C. Similarly, when people go for a few days without sleeping, they get sick.
D. The processes take place to stop our brains becoming loaded with memories.
E. That's why students do better in tests if they get a chance to sleep after learning.
F. "Sleep is the price we pay for learning," says Giulio Tononi, who developed the idea.
G. Tononi's team measured the size of these connections, or synapses, in the brains of 12 mice.