It indeed is heart work and not hard work. I'm hyperactive by nature. People often ask me "Aren't you tired? You're just off a 24-hour shoot and still going all guns blazing?".
And my reply to them is "When I enjoy what I do, it doesn't feel like work. It's more pleasure. Now, why would you want your heart to stop wanting pleasure for itself?"
It's a lovely game where your heart and mind convince your other senses in a way that you give your all and still not feel tired. So much of it is all in our head and heart.