Sleep is a normal, indeed essential part of our lives. But if you think about it, it is such an odd thing to do.
At the end of each day we become unconscious and paralysed. Sleep made our ancestors vulnerable to attack from wild animals. So the potential risks of this process, which is universal among mammals and many other groups, must offer some sort of evolutionary advantage.
Research in this area was slow to take off. But recently there has been a series of intriguing results that are giving researchers a new insight into why we sleep and what happens when we do it.