He or she is having trouble sleeping
Children often have trouble sleeping, but if it becomes persistent and extends beyond the point you would have imagined they would grow out of it, there may be a deeper issue - potentially even a mental illness.
A study at the Norwegian University of Science and Technology looked into this link. Their study stated, "it is common for children to have periods when they sleep poorly, but for some children, the problems are so extensive that they constitute a sleep disorder. Our research shows that it is important to identify children with sleep disorders, so that remedial measures can be taken. Sleeping badly or too little affects a child's day-to-day functioning, but we are seeing that there are also long term repercussions."
So, if you think your child should have outgrown their sleeping troubles, don't wait before you visit a doctor. They could be suffering from anxiety, depression, or a variety of other mental illnesses. Should their sleep issues persist, they could also develop other negative conditions not currently present.