But the good news is, it can be treated.
Around 10 per cent of children still wet the bed at age eight, and it can be really upsetting for them and their parents.
But this new research shows there's nothing seriously wrong with children who wet the bed, they've just got some nerves in their head which haven't grown as fast as other kids'.
Help in the future
Another possible reason for bedwetting is that some kids don't have much of a certain chemical, which stops your body making wee in the night.
Children with this problem can be given extra helpings of this chemical, specially made by scientists, which can make a real difference in how often they bedwet.