Part of the life of a little guy is completetly wearing yourself out and then completely crashing. Sometimes that crash happens when and where it's supposed to (like in the bed at naptime), but other times it's sudden and wherever you might be (like on the floor at what was supposed to be the end of nap time).
As adults we face the same things. We don't often get the chance for really good naps (though I set aside Sunday afternoons as guaranteed nap time) and it's hard to recharge once your batteries are low.
I love the way Eugene Peterson's The Message puts it in Matthew 11:28-30: "Are you tired? Worn out? Burned out on religion? Come to me. Get away with me and you'll recover your life. I'll show you how to take a real rest. Walk with me and work with me--watch how I do it. Learn the unforced rhythms of grace. I won't lay anything heavy or ill-fitting on you. Keep company with me and you'll learn to live freely and lightly."