Tramadol and sleep

I am having knee replacement surgery next Friday. The pain is severe enough, the doctor put me on tramadol and said it would also help with sleep. It did for two nights, then stopped working.  I stopped taking the pain med after four days, but my sleep is still mostly gone. Two nights ago it was six plus hours, but last night it was one hour. I have stopped feeling tired at any time during the day or night.

The pain is better because I am doing pre-operative exercises each day. 

I suppose I will get better, as whatever the tramadol did to my brain wears off and the brain heals. 

In all honesty, I wonder if not sleeping will cause any kind of problems during the surgery and recovery. Any thoughts on this?

Thanks

NG