The Reason: You may not feel like you're walking through the Sahara, but there's a good chance you're mildly dehydrated right now, and even mild dehydration makes you feel sleepy and sluggish. "Many of us walk around dehydrated without realizing it," King says. "Dehydrated bodies trap toxins and encourage water retention -- a natural defense against the chronic 'drought.'" Remember, your body is about 60 percent water, and there's no way it can function at its optimal level during a drought.

King's simple test for dehydration: Pinch the skin on the back of your hand and hold for three seconds. When you release, if the ridge from the pinch remains for more than a second, you're probably dehydrated.

The Fatigue Fighter: Next time you're feeling drowsy, chug a glass of cold water for an instant fix. Then, take a look at how much water you've been drinking. According to Dr. King, you should aim to drink half your body weight in ounces of water every day. If you weigh 150 pounds, that's 75 ounces of water (about 9 cups). If you want something more pleasing to your palate, snack on watermelon, which is about 91 percent water.

The Reason: As you may have guessed, breathing is an essential part of being a fully functioning human being. With that in mind, your terrible posture can be enemy No. 1. Slouching puts uneven pressure on your spine and causes different muscles to overcompensate and tighten. This makes it hard for your lungs and diaphragm to work properly, and labored, shallow breathing is a one-way ticket to zombie town.

The Fatigue Fighter: According to King, you can take in up to 30 percent more air if you pretend a string is pulling you up straight. Lift your rib cage away from your hip and push shoulders back to keep your blood circulating and oxygen flowing to your brain. Do neck and shoulder shrugs in your car to reenergize on your morning commute.

The Reason: We have routines for a reason, but constantly going through the same motions day after day can put you in a rut and your brain on autopilot. When you're completely zoned out or bored, your nucleus accumbens, a central part of the brain's "reward center," is seriously lacking in the feel-good chemical dopamine that encourages drive. So dragging your feet (mentally and physically) and having zero motivation or stimulation can leave you feeling drained -- even if you haven't really done, well, anything.

The Fatigue Fighter: Try something new. When you do something novel, your brain floods with dopamine, that feel-good hormone that influences alertness, attention and concentration. Something as simple as ordering a new, innovative dish, grabbing coffee with a new coworker or turning a chore or work-related task into a time challenge (first one to finish doesn't have to buy a round at happy hour) can give you a much-needed jolt.

The Reason: Similar to bad posture, stress can lead to tense muscles and shallow breathing, which leave you feeling lethargic. To make matters worse, focusing on one thing for a prolonged period of time, like that looming deadline or a presentation you've been staring at for hours on end, causes your brain to use up so much energy that you feel physically drained. Researchers at the University of Toronto Scarborough found that mental concentration is much like a muscle -- it becomes tired after sustained use and needs a rest period before it can recover.

The Fatigue Fighter: Instead of stewing in your stress and digging yourself into a nerve-wracked hole, force yourself to take breaks. Studies found that during the day, we move from alertness to fatigue every 90 minutes, but instead of listening to our bodies' signals to slow down, we shut it up with sugar and caffeine.

To get a boost of energy, work uninterrupted for 90 minutes, then take a 10- to 15-minute break. And no, updating your Facebook doesn't count. Leave your desk to chat with a coworker or go outside and smell the fresh air.

The Reason: We all know some kind of energy vampire: The whiner, the naysayer, the chronic blamer, the jabbermouth, the frenemy ... dealing with these people is emotionally draining, which drags you down and depletes your energy. Yes, it's literally exhausting to be around these people.

The Fatigue Fighter: You can't choose your family or your coworkers, so your best bet is to play defense. Set time limits for recurring sob stories ("I only have five minutes"), hop on a "phone call" or excuse yourself to the restroom to avoid blabbermouths, and never encourage or reaffirm blamers.

If you're more of an offensive player, try calling up the friend who always makes you laugh -- she actually gives you energy. Ever notice how invigorated you feel after a good laugh session? Laughing improves blood flow, wakes up respiratory organs and suppresses stress hormones -- consider LOL-ing a little packet of aerobic exercise without the sports bra and sweating. If you can't arrange a date, choose the next best thing: some quality time on Funny or Die.