Nice way of telling MIL I don't need help...

My baby is a few months old. My MIL is always inviting herself to come over to "watch the baby so I can nap". It's a classic Everybody Loves Raymond scenario... always just coming right over. Well, she's more passive aggressive about it, which is probably more irritating. She sees my baby like once a week already. My mom comes over a couple weekdays to help me. That's already enough relief right there. I don't need help EVERY SINGLE DAY. I don't have a newborn anymore. My baby sleeps very well during the day (5-6 hours total), so I get a lot of sleep, too. What is a nice way of telling my MIL that I don't need help every single day and I'd rather just be by myself?