I suspect there might be something else at work here. Ask yourself this question: if you take a 20 minute nap at work, will that count as part of your working hours or not?
Even at the places where employees are not expected to rigidly conform to fixed working hours, there's an expectation to put in a certain minimum of hours. I'm not talking about producing a certain output: even in the most "goal-oriented" places I've worked at, there was still an unstated, tacit expectation that you were supposed to put in a certain minimum of effort, regardless of your results.
What I'm getting at is that if you have employees who nap and those who don't, there will almost certainly exist an expectation that those who nap should put in extra time equal to the length of their nap. Therefore, those who do nap will lose some of their free time.
If you're single, that could be an acceptable bargain: instead of sleeping for (say) 8 hours at night, you would have a 20 minute nap at the office, stay 20 minutes longer and then sleep for 6 hours at night. The net gain would be 80 minutes, which isn't bad at all.
On the other hand, if you're a family person, then that's 20 minutes less with (say) your kid after school.