Well, if I had a "do not disturb / sleeping"-sign, then it's the guy who wakes me up that need to come up with the excuses. It doesn't happen often that I sleep at work. But it has happened, and I've never needed any excuses. That's because while I've been sleeping on normal office hours, I haven't done it on working-time. If I am at work and sleep, then it's by definition my free time. Ie, I will compensate it by working a bit longer. Same thing if I feel like taking a 45 min coffe break (which happen now and then).