Should I be expected to babysit my grandson (who lives with me) every night so his mother can go out?

I have my 25 year old divorced daughter and her 3 year old son living with me. My daughter goes out every night after her son is in bed and stays out until 4 am and expects me to babysit since I am home anyway. He does not always sleep through the night and I am working early in the morning. She is not working or attending school for the summer. When I refuse, she goes out anyway. Am I being unreasonable to expect her to stay at home at least a few nights?.