Masturbating is not abnormal, even in married couples. Maybe it's a little unusual that she does it in her sleep, but probably not entirely unusual...some people walk in their sleep, and on Ambien some people even get up, get dressed and drive a car, but aren't really awake. Don't take it personally, it's just physical release.
yes i Agree masturbating is fine but when she says she is tired and has to go asleep and then proceeds to turn over and masturbate, it sends the wrong signals and creates more problems than it solves. I can now understand why she does not need sex too much as she is satisfied by her own actions, which just leads to the husband going elsewhere to be satisfied, even though he does not want that situation
If a husband is dissatisfied in his marriage, he should tell his wife, not "go elsewhere to be satisfied." If she is concerned not to lose you, she will take this seriously. It is a cheap justification (not to say immoral) to use a wife masturbating in her sleep (you say she says she knows nothing about it) as an excuse to cheat. Tell her it's a problem, work on ways to address it. If she can't or won't change, then consider a divorce if it upsets you that much. But don't go off and cheat and then blame it on her.
I can identify with wanting to masturbate instead of having sex, especially when I'm tired. Masturbation (at least in my experience) doesn't take a lot of energy, while sex usually ends up in various positions, takes quite a while, etc. She can do it however she wants, whether it's going all out with a dildo, or simply rocking back and forth against a bunched up sheet to lightly stimulate her clit. She may also not have the energy to feel like she can't satisfy you (for ex. don't expect her to go down on you if she's tired). Maybe you should try to slowly get her in the mood (especially if she's already masturbating)... rub her back, arms, and so on in a non-sexual but sensual way.