As a user, unless I'm logging into and using your product everyday, I'll definitely forget my login id in a few days and then go through the headache of trying

As a user, unless I'm logging into and using your product everyday, I'll definitely forget my login id in a few days and then go through the headache of trying to find my login id etc leading to a poor experience. So unless you have a very good reason to do something different, stick to login ids as email ids