“Energy is always constant in an isolated body, it is said to be conserved over time. It neither can be created nor destroyed, it can only be transformed or transferred from one form to another.” – Theory of Energy Conservation
This is the first law of thermodynamics and holds true for all the closed systems. Our universe is one such closed system and we are a part of it. Every person has a powerhouse of energy stored in their body and this energy can be lent out only in changed forms. So when we actually feel low in energy, it is because we have spent the energy into something else and our body needs more of it.