What Causes Addiction To Masturbation?

Hello, My wife masterbates nearly every night (19 out of 21) regardless if we had sex or not that evening. When I first questioned her about it,,,, she became very defensive and denied it vehemently, I resorted to recording her activities and verified I was not mistaken ( She does not know yet ). We have a robust sex life and she claims to be very satisfied with it. We have virtually no limits to our sexual escapades with the exception of her performing self stimulation during our sexual encounters. I have encouraged it repeatedly as I find it a great turn on but to no avail. Her response is almost that of disgust. She works earlier than myself so she goes to bed an hour prior to myself. It is during this time she indulges herself. I now question if this is due to my personal shortcomings in our lovemaking and she doesn t want to hurt my feelings. I am now having serious sleep and eating issues as a result. I am 48 and my wife is 46 and we have been married 24 years. Any medical insight you can provide would be greatly appreciated.