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.