If we could approach our health like a shopping list, most of us would probably wish for a lot of the same things: to avoid illness, to get more sleep, to maintain a healthy weight, and to live a long life while keeping our wits about us. 

Another desire we’d likely share would be linked to all these aspirations: to have more energy. After all, no goal is really achievable if we don’t have the energy we need to begin it or to see it through; and few things are more frustrating than spending the day feeling like you’re running on empty. 

Here’s what most commonly depletes energy levels, what restores them, and the difference between temporarily diminished energy levels and something more serious.