I can't tell you for certain. I have never had a cornea burn from sun bathing, fishing, swimming, etc. So I think it likely all or most is blocked out.
Its more important to worrry about how to protect your eyes outdoors with your eyes open.
Windows block out most UV.
Your solution: hat with bill, SPF 50 waterproof sun tan lotion, UV blocking sunglasses.
JCH MD
Is safe to say keeping your eyes close blocks out UV A, B and C? And if so do you know what the % is? Also if you get visible light in your bedroom from outside does that contain uv rays or does the sun need to be facing your window in order to get it? I live in PHX, AZ and it gets very hot here in the summer.
A quick internet search confirms that the figure "less than 25%" is thrown around often. HOWEVER most of these were commercial websites that were selling sunglasses or eye protection for tanning booths.
I did find one study that indicates that while the eyelids don't block all the visible light they do block most UV.
I have never had a corneal burn from sunbathing with the eyes shut.
Sunbathing is not a good idea, ages the skin, causes skin cancer, cataracts, macular degeneration.
JCH MD