A Bad Night's Sleep
If we view the body as a battery, then sleep is the most obvious charger. Our bodies rely on the rest provided by a good night's sleep to get through a busy day. One bad night can throw the whole system off.
When it comes to sleep, both quantity and quality matter. Eight hours seems to be the needed amount of sleep for most people. While some people may need a slightly higher or lower amount, it's important to get that quantity of sleep each and every night. If not, you'll be a walking zombie without the energy to pick up a pencil. And don't think that you can hold off until the weekends to sleep in; studies have found sleeping longer than usual can result in even more fatigue [source: Harvard].
Maybe you carved out the time to get your full eight hours, but you tossed and turned all night. To fully recharge, your body needs to reach a state of deep sleep, which is believed to play the biggest role in restoring energy. During this state of sleep, your blood pressure and pulse rate drop, as compared to the more active stage of REM sleep when you dream. If you're not getting enough shut-eye and feeling the effects in the morning, try to evaluate what choices might be getting in your way, like late-night caffeine.
Sleep's the foundation for many of the items on this list. Just try, for example, to face the office without it.