Just tested it in my bathroom in total darkness. Wearing a thin white undershirt and the lights off I press the shirt up against the rear sight, they'll glow through. If it lays like it would if I was normally wearing it, it doesn't at all. So unless you're in 100% darkness with a thin white t-shirt and you intentionally press your shirt against the rear sight, it's not an issue.
SIDE NOTE: It's funny, many people claim (maybe not so much anymore) night sights will give away your position in a low/no light situation. What I have found with sight is if someone can see the glow from your night sights, they can also see you.