What if...Sleep Meter

Here is a thought that crossed my mind in my worn out exhausted body (due to school and such). What if when you closed your eyes and slept you could see how much energy you had in your body in like a percentage. Similar to a battery meter on a cell phone, you will have a percentage of a blank battery green for how much energy you have. This way you would know if you needed to sleep for a long time or if you should go back to sleep. Also you would be able to realize how much energy you had. "Oh I can't do that today. I only have 45% energy left." Would be similar conversations that went on between peers. You would only be able to see it if you were asleep, then you would be able to subconsciously make decisions about your sleep habits from being able to tell. There are a couple details that don't work out, but it's only a dream.