That's why it's called a mental illness. Everything is in your head. Well, The effects of the mental issue affects the physical one.
Say you're depressed. You don't want to get up from your bed and just want to slouch. That's terrible, But the reason is that your brain is telling your body that. Everything in your body is controlled by the brain (I think? At least physical depression or something like that) and therefore physical effects of anxiety and depression are caused by (and are in) your head. The defenition of mental illness doesn't say anything about physical effects.
I am always nervous but I learned that somethings are out of my control. I do control how my day goes and what I do. I could be sad about how I live but I take comfort in my God and how blessed I am.
I eat breakfast and dinner when some people have nothing. I have a place to sleep where people sleep outside. I have my family and people live alone. I try to help people less fortunate than me.
I leave behind my anxiety and depression and they have no power over me.
As someone who's struggling with anxiety and depression right now, As many people do, I can say that yes, It is mainly a mental health illness. But there's also more to it than that. It effects your mental and emotional health, But it also makes you physically exhausted. I know someone who got physically sick and had to go to the doctor because their depression prevented them from doing any activity for a prolonged period of time, Which caused them to get muscle aches, And eventually develop a fever.
For one thing, Saying it's all in your head implies that it isn't quite real. And it is. I would know. For another thing, There are physical symptoms of anxiety and depression that, While they may originate in your brain, They can effect your health in other ways. Honestly, Saying that a person's mental disorder is "all in your head" isn't just inaccurate, It's rude.