My daughter went through a stage where she will wake up after 4 or 5 hours and just cry when she was around 3 years old. She was not really awake but cry for no apparent reason for 1/2 hour. Asked doctor and the doctor say it could be night terror and we should just let her cry. After a while she will fall back to sleep. According to the doctor it is the period when she is going into deep sleep. This went on for nearly 3 to 4 months and after that it is just stopped. Another way the doctor advise us was to try to wake her and bring her to go pipi when she has slept close to 4 hours. This way does not always work as she does not want to wake up but when she wake up and go pipi, she won't cry again.