Have you ever taken on extra work despite a packed schedule, just because you didn't want to disappoint your colleagues? Agreed to an appointment when all you wanted was rest? Or tried to soothe a loved one's emotions, even when you yourself were hurting?
What sometimes makes you tired isn't having too much to do, but the feeling that you're not allowed to refuse. You're afraid of being seen as selfish, unapproachable, or not caring enough. So you choose to give in, then convince yourself that as long as everyone is happy, your own sacrifice is insignificant.