This is because snoring means you actually breathe in less oxygen while you're asleep, so you don't get such a good night's sleep and your body doesn't recover properly from the day.
So children find it harder to concentrate at school the next day.
Scientists think this is such a big deal, they're telling parents to watch their kids sleeping, to see if they snore.
And if they do, they should get medical help. Snoring can be caused by about 50 different conditions, but most of them can be easily treated.