Scratching Noise In The Ceiling

There's a strange problem in my flat which has been bugging me for a while. Every now and then I wake up in the middle of the night because I can hear strange scratching sounds coming from the ceiling. It sounds as if someone is dragging a set of keys along the floorboards above, or as if there is someone trapped up there, desperately trying to claw their way out. It's all a bit spooky.

The most logical explanation seems to be that it is mice - but I don't see how they could make such a noise. We had mice in our flat a while ago, and they are tiny, making barely a sound. Do they have long fingernails?