It is most likely field mice, but the previous writer is correct that you need to figure out how they are getting in before they really become a serious problem. We live out in the country and field mice are a constant. I love animals and would not kill them, but I will be the first to admit, once they establish themselves, they can be very destructive. They love to chew wiring. They chew clothes. They reproduce like crazy.
Field mice can get into the tiniest of places. Anything bigger than 1/4" and they can squeeze right through.
Peppermint oil seems to repel mice to a small extent, but the bottom line is - find the holes they are getting in and seal them up. To catch the ones already in there, I recommend Hav-A-Hart live traps. I have tried every humane trap known to man and they are all a joke except the Hav-A-Harts. Don't use a generic one, use the actual HAH brand. Put a little peanut butter on the trip plate and the trap snaps shut, capturing the little one alive and well. You take him out and release him in the woods.
If you do set a live trap, be sure to check it twice a day so the mice don't die from dehydration (which occurs in less than 24 hours due to the stress).
If I sound like a rodent expert, I actually am. Hmm, not sure that's something to brag about!