Tough one Ryan.
I thought everybody masturbated though right? I know me and my husband both do at different times- but I guess the real thing is you feel shjes choosing this over you. Maybe you could try some of the great suggestions of whise shes doing it approach her.....she can surely not deny it if you are looking at her mid way!!!!!
I think maybe she either just wants some down time...or maybe she feels a bit pressured to have sex so it puts her off then when she thinks your asleep and that stress of feeling pressured is gone she lets herself get aroused and does the business??
You mentioned you are the more highly sexed...and you mentioned that you have talked about it and she has improved things and then they go back to dry spells again. so maybe its her way of coping??
Either way I hope you 2 resolve it - sounds like you have a great marraige besides this so I wish you all the sex and happiness in the world!