“Tired” sometimes serves as a convenient way to label your state when you’re in a crummy mood, but most of the time, the weariness precedes the feeling. The effect of your sleep on the rest of the day is more than tangible, and you have to acknowledge that as you grow tired later in the day. The best route is not always to power through and continue to labor. Fifteen minutes of shut-eye would benefit you far more in the long run.
The only reason you don’t nap is that you’re too vain. You are worried you will look lazy or like you don’t want to work by taking a nap. This isn’t present only in your behavior in the workplace—even in your own home, when nobody is around, you force yourself to stay awake and continue to work. Is your God going to condemn you for having slept to continue being industrious? I would hope not. Do what you need to do and if you’re so unfocused because you’re exhausted, then to hell with your work. Get focused. Without focus, attention, and intention, you are useless.