And when Jesus was awaiting his fate, he kept finding his followers asleep, and chewing them out for it. Which I don't think was meant to say we shouldn't literally sleep--but it was a good illustration of how easy it is to fall asleep in the sense you're talking about. [In your comment a little while ago.] It's like people no sooner find something that strongly awakens them--than they try to find a way to sleep on it.
Perhaps it's that spiritual 'energy'; it feels great but it wears us out! (The reason for yoga, to build up our capacity? Sounds plausible.)