My mother-in-law does not leave my newborn baby alone even for a second. While I know she wants to take care of the child, how do I ask her to leave me alone with my baby for sometime in the day?

I suggest, tell her politely that mummy I think you should also take little rest and we too shall sleep for a little while. Or you can tell her that mummy I think that from now on after feeding the baby, we both will rest for a while as I hardly get any time to rest or am not able to sleep properly during nights, so every day I will sleep during the day. MIL's are very smart, she will take a cue, and you will get time for yourself.