Do naps leave you feeling less than refreshed? You’re probably napping for too long. Sleep inertia – waking up from a nap feeling even more tired, out of it, or even nauseous – can happen if you nap for so long that you get into a deep sleep. The best way to prevent it, experts say, is to power nap – dozing off for about 15 minutes. That way you won’t get into that deep sleep phase that can leave you groggy when you wake up.