sleep less nights

MyRah is 18.5 months now, she is always clinging towards me and wants me beside her all the time. She has become very cranky in her behaviour.Main problem with her is that she doenst sleep throught the night. She normally sleeps by 11.30pm and she is in deep sleep itill 3am after that she is keeps crying every 15 mins. She is very playful and gets Since me and her father both being working parents we too need proper sleep but with this nature of MyRah makes us upset.
I feel would she be having some stomach aches, or something else...
Please give me some solution.