Just a point to consider for those who think lack of energy is all in the mind.You do not actually have much mental control over how much energy your muscels can use.This is a bit like smiling , you cant fake a smile because you dont control those muscles.Anyway the body keeps you from using energy all at once and spreads it out over time, so that you don't use it all entirely.In extreme situations of stress you can gain access to hidden resrves of strength when extra energy is realsed by your body for use.The potential energy in our body is much higher than what we normally use.If for example you recieve an electric shock this can send you flying across a room.This is due to the muscles contracting with all their force( just dont let it happen in the palm of your hand as it will grip onto the the electric source and wont be able to let go)much more than you could ever summon mentally.
The point being that energy levels can vary between humans, and is not governed much by will power.It stands to reason that some people will suffer from having less energy than others and this can be worsened due to other factors.