Long nights are usually the best way to put a damper on the hormone levels. Once the hormones drop down to non-breeding levels the masturbation will stop.

I wonder if it's because I gave them a sea grass mat that they have been chewing on. Is that like paper on the bottom of the cage?

I have had them on thirteen our nights. I will try to make the dark an hour longer.

Chewing on a mat would probably contribute to the problem.