It sounds TYPICAL of MG (I get it all the time)-

Think about it - when you are in church, a restaurant, or a theater, you are usually in a seat that only supports about 1/2 of your back, and NONE of your head. You ever stop to think about how much energy it takes to hold your head up? An awful lot, that's how much.

And the head/neck is the only non-bilateral part of our muscular structure - there isn't really anything to counterbalance it, it's basically a heavy ball on a short stalk.

That is why I carry a "camp" rocking chair in my car at all times - it has a full back on it (well, for me anyway - I'm pretty short), so my head is always supported. And I tilt it back so that the weight of my head is taken off of my other muscles. I still get tired, but it doesn't happen quite as quickly in that chair.