Interesting things happened today: I was so tired, was barely moving, had zero energy. When I sat down to meditate the lethargy completely went away, like it wasn't even there!

There was clarity and energy almost instantly, as I sat down. Now that I'm not meditating, the yawns are back and the feeling of tiredness is slowing coming over me.

How is this possible? How does it work? How can I maintain this clarity and brisk feeling throughout the day without going into a slump?

Is food related to the energy level as well?