It depends on how sensitive you are, a smart watch would ruin me but I have sensitivity so severe that it's literally life threatening not too have shielding on my electronics. In fact funny this topic is brought up today because I am all EMF'd up just from typing too much but in order to do that I need to be typing so much for so long that my finger hurt and it was an entry for a writing project I'm doing. From a phone with a normal cell signal going out of it without wifi enabled I'm alright as long as the phone is away from me, I'll only get sick if I hold it to my head for too long. Usually with lower grade over exposures like i had today I'll get brain foggy, my vision will get somewhat cloudy, I'll feel some slight tension in my head/neck, I'll get a mild sense of doom and feeling like I want to cry, feel disconnected from my surroundings, and some facial heating. As the exposure gets worse it and the crippling brain fatigue it comes with will get worse. My environment is also bad, really high end read outs fro wireless radiation in my current space and I can't move either so I'm constantly getting a steady dose of the stuff regardless. I survive it as long as I'm at least a couple rooms away from a wifi transmitter.
On the topic of this thread about "brain fatigue" though it's different from regular fatigue, like right now I am physcally fine and today I'm actually feeling a bit stronger but it's distinctly an exhaustion of my brain itself that leads me feeling faint and weak overall. It's very difficult to describe that one.