9 month old wakes up same time every night !

Meiling

Does anyone or can anyone tell me why my 9 month old little girl wakes around 3:30am every night same time and takes an hour to go back to sleep. Need to rock mainly to sleep. She's just awake and not upset or hungry or anything. I use white noise and rocking chair and eventually she does go to sleep but it's a ritual nearly at this stage she wakes up.