One thing that I am better about now than in the earlier days of my marriage, is not feeling like I had to fight all of my husband’s battles. It’s not that I wouldn’t defend him, but that I don’t think it’s necessary to speak for him. He is his own person, and especially to his own family, is capable of expressing himself, should they choose to ask him a direct question. Often, such conversations are a subtle form of gossip or an attempt at manipulation. So I prefer to just be uninterested in a person’s problem with my husband than attempt (and probably fail) to be a mediator.
**So if your MIL doesn’t like that your husband is going to bed early, why would she need to complain to you? My response would be something like, “hmmm…” (and nothing else) or “hmmm, I guess you’d have to talk to him…” or “hmmm…I don’t know, I guess he prefers to go to bed earlier now…” The “hmmmm…” is to make that point that you don’t really care and haven’t thought about it before and the rest is to point out that he is a big boy who can handle his own bedtime, and that it is not anyone else’s business. After all, why on earth would one adult’s bedtime be a topic of conversation between two other adults? **
I’m sure there are other conversations than bedtime, but I would avoid the trap of defending him. It is too easy to guess incorrectly what he would say, or to say something that doesn’t actually help the situation. Better to just avoid those conversations and change the topic to more pleasant things. (and talk to him about having some short, non-argumentative, vague answers like, “I just prefer it this way now” and, “it just works better for me this way now” etc.)