I want to sleep but i cant...

Hi... I stay home taking care of my babies one is 18 months and the other is 8 months. The thing is that I don't know why they don't sleep the whole night.

They used to go to bed around 11pm and wake up the next day, but not anymore. They still going to sleep around 11pm but they wake up again around 1 am fully recharged. They start screaming, crying so loud that I have to take them to the living room so they dont wake up daddy because he has to work.

But sometimes I feel like they got out of control. They dont go back to sleep until 5am sometimes they change the schedule, go to bed around 1am and stay up until 6am even 8 am. I really dont know what else to do.

I give them a nice warm bath, warm milk, change diapers, read stories, everything is quiet (off course everybody is sleeping) but them. Sometime they even take turns, while one is sleeping the other one isn't and vice versa or both are awake.

I'm tired all the time cuz I cant sleep enough and they are ready to go again.

Can someone tell me whats to do????