I know, this can all be really confusing at first! I'm glad the blurriness goes away when you shut one eye. That means it's the MG.
The reason your eyes have blurriness or double vision is that the muscles in and around one eye are weak to varying degrees; same with the other eye. Since all the muscles of both eyes are not working the same way and in the same strength, they are "focusing" differently. And, yes, that weakness can come and go throughout the day depending upon how weak the muscles are. There are lots of muscles in and around the eyes!
How about a metaphor? The eyes are kind of like ships in rough waters (water being the muscles). If the waters/muscles are calm (and not weak), the ship/eyes are relatively okay. No blurriness or double vision. The more turbulent the waters are around a ship, the more the ship is in motion and cannot stay steady. So the more you use your eye muscles, the more they weaken and the worse your vision gets. Sorry, that wasn't a great metaphor but I hope it helps.
What is making your muscles weak is an autoimmune process. That's why steroids, aka medrol, works. Steroids suppress the immune system so that you create less antibodies attacking your neuromuscular junction.
Anything that makes your immune system worse would theoretically make your MG worse (increase antibody production). Being tired, like you said, makes it worse and can, ironically, make you feel even more tired!
It's not the "sunlight" that usually makes people worse but the ambient heat of the sun. Any heat will make you worse. Infections, lack of sleep, stress, surgery, etc. can all make you worse.
So, the opposite is true. Staying calm, being happy, staying well, sleeping well, staying neither hot nor cool will all make you feel relatively better.
When I've been typing on the computer too long, my arms, especially my right one, can get much weaker. They can get achy and in pain. Why? Because overuse of muscles, even in people without MG, can cause secondary pain. Talk to anyone who is a physical therapist! When I've overused my back muscles, they can get so weak that they cramp up and spasm. There have been times when I can't even stand or they are bad even when I'm in bed. Now that's the extreme but that's what can happen when your MG gets worse. Right before my MG crisis, my back was so bad I couldn't walk. Those back/chest muscles are called the chest wall muscles. You need them to be strong in order to breathe!!!
So if any of your muscles get achy or cramp up/spasm, it should serve as a "warning" that your MG is getting worse and you need to rest more!!!
The more you do, the weaker your muscles get. Then you need to rest. Just listen to your body and it'll tell you what it needs.
Sometimes putting a cool washcloth on your eyelids or sitting in front of a fan with your eyes closed can sometimes help. Coolish conditions can make MG better but not too cold because cold can make it worse. It's a real balancing act!
Make sure you go to and read all about drugs that might make MG worse. Benadryl is one of them. I can handle small amounts of it but everyone is different.
Have you ever been checked for allergies? People with sinus infections sometimes have hidden allergies (respiratory or food). Do you have a good allergist? Someone I know has respiratory allergies and uses Astelin. It's a nasal spray but it's also an antihistamine like Benadryl.
Just keep on reading. I know this is hard but it's doable. I hope you have a good neuro who will figure all this out and help you.
Annie