Humans store energy within our bodies via a chemical reaction and when needed the energy is released via a chemical reaction. Humans process of storing and releasing energy is known as the Krebs cycle.
Car Batteries
For the traditional combustion engine automobile, when you start your car the battery discharges electrical energy which is used to get the motor started.
During operation the motor (now running on petro energy) the battery is re-charged with electrical energy converted from the mechanical energy supplied by the motors rotation. This chemical reaction of the battery is reversed preparing it for the next time a charge is needed to start the car.
If you don’t properly maintain and charge your car battery, you can encounter times when your car won’t run.
Human Bodies
Similarly the energy stores of the human body need to be restored occasionally (or you are in danger of stopping).
Food provides the chemicals the body needs for the Krebs cycle and this is a very basic way we recharge ongoing (i.e., by eating). Of course the myriad of chemical reactions that go on within the human body are a bit more numerous and complex than just the Krebs cycle. The complex series of chemical reactions occurring within our brains (7 days a week, 24 hours a day) all need some form of recharging, whether through addition of an energy source or just through rest.
Different Levels of Recharging Needed
How much and how often and what kind of recharging each of us need is unique to each individual.
Sleep is one of these recharging processes and although there is a medium desirable amount of sleep each day (currently assessed at about 8 hours) on a person by person basis this need can vary dramatically from light sleepers (2-3 hours a day) to deep sleepers (needing up to 12 hours a day).
Interacting with others
Whether our basic personality is introverted or extroverted also has a major impact on the need to recharge.
An introvert who spends a few hours in extroverted activities can find themselves needing hours of alone time to recharge while an extrovert can participate in hours of robust social interactions with others and needs little rest time afterward.
Self Awareness
It is of great personal value to be aware of your limits and pay attention to your personal need to recharge (whether through sleep or time spent on your own or other relaxation activities )
To operate at optimal levels and enjoy yourself, and interact best with others an understanding of and acting on your personal recharging needs is required to stay healthy, happy and well rounded.