During last cold episode i was taking tylenol 500 mg at night for few days. i noticed getting very good night sleep. is it ok to keep taking one tablet every night for long term?

No: In general, it is not ok to always need to take a chemical (a tablet, capsule, elixir, etc...) in order to get to sleep or get better sleep. That can be done temporarily, but should not be the case on an ongoing basis. However, some people's psychiatric or sleep disorders are severe enough (unfortunately) to require ongoing medication. That said, Tylenol used chronically can damage the liver.