“What have I done to deserve this?!”
Said Amanda Palmer as a frat boy tried to shove his head into her crotch. Palmer, an up and coming musician, was spending her days as a living statue in order to help pay the bills. She would spend hours in a bridal costume on top of a box in the middle of Harvard Square and would not move an inch.
Some people would insult her, others would try to make her laugh, and some would threaten to steal the money she had been collecting. But through it all, she remained completely still.
Then there were the countless other distractions she dealt with. She couldn’t move her eyes, so she had to ignore all of the interesting things passing by. If she had an itch, she wouldn’t be able to scratch it. And if a bug just happened to land on her face, she had to wait patiently until it left.
All of this discipline completely drained her. She could not work for more than 90 minutes at a time, and could only do it for a total of 3 hours a day. And when she got home, she was completely exhausted mentally and physically – even though she barely moved! 
Draining MENTAL ENERGY
At some point in your life, you have probably experienced the same phenomenon as Amanda Palmer. You struggled through a stressful day at work and for some reason you feel completely exhausted when you got home. You may have spent the entire day at a desk, but for some reason your legs, your arms and your body all feel completely drained.
Why is this? Why does our whole body feel physically exhausted when we’ve only been exerting ourselves mentally?
To try to understand this, researchers ran experiments that would test someone’s self-control in one task (such as holding in their emotions), then immediately test it in another unrelated task (such as squeezing a handgrip as long as possible).
What they found was that regardless of the order of the tasks, how they related to one another, or how “disciplined” the person was, they always performed worse on the second task than they did on the first. This pattern continued if they were to do a third or fourth task as well. Every time they exerted their willpower, they had less of it left for later tasks. 
WHAT THIS MEANS
We tend to believe that the energy we use is different depending on the task. We have one stock of willpower dedicated to resisting the snooze button, another dedicated to eating healthy, another dedicated to our professional productivity and yet another for being cheerful around our family even when we're in a bad mood.
But these findings show that this is not the case. We actually have only one “reserve” of willpower that we use every time we make the harder choice. Whether that choice is resisting a cookie, putting on your running shoes, or dealing with an annoying coworker, you are pulling from the same willpower reserve. 
So this means that when we come home after a stressful day of work, we will be less likely to have the energy to cook ourselves a healthy meal. If we’re on a diet, it will be harder for us to work up the strength to hit the gym. And if we’re up late studying for an exam, our kitchen is less likely to be cleaned.
HOW TO TELL THAT YOU ARE LOW ON WILLPOWER
When we’re low on willpower, we immediately fill our minds with thoughts of self-doubt and guilt. We wrongly think that there must be something wrong with our character, when this simply isn’t the case. This is like saying to an athlete who has endured a full day of physical training, that she should feel guilty about being tired.
You are simply low on mental energy the same way the athlete is low on physical energy. Being aware of what is happening when you are low on willpower is one of the best ways to push through one of these moments of weakness. If you recognize that you are simply low on energy, you can do what's necessary to restock your willpower and be ready to take on another challenge.
The clearest sign that you are low on willpower is that the overall intensity of your feelings grow, you start to lose control of your emotions, and you start to crave more sweet food. The reason these things are happening is because your brain is recognizing that it's running out of energy and regulating emotions is the lowest on the "self-control priority list". Then it will give you cravings for sweets because sugar is fastest way to give your brain more mental energy.
SO WHAT CAN WE DO?
If you find yourself in this state, there are several things you can do to restock your willpower and get ready to take on challenges once again.
1. Eat Fresh Fruit
Fruit will give your brain the sugar that it is craving without causing the "spike and crash effect" that other sweets like candy or soda will. Some good choices are bananas, blueberries, apples and cherries which will all successfully restore your willpower. 
2. Relaxation Exercise
Quick relaxation is one of the best ways to clear your mind and restore your willpower. All you need is 5-10 minutes to become refreshed and get back to tackling challenges. Just follow this relaxation exercise from Stanford Health Psychologist, Kelly McGonigal: 
Lie on your back
Elevate your legs slightly, or do what is most comfortable.
Close your eyes and take deep breaths
If you feel tension in any muscle, contract that muscle tightly for a few seconds then relax it - repeat for all tension in body
Then just relax for 5-10 minutes in the knowledge that there is nothing you have to do besides relax and enjoy yourself
- If worried about falling asleep, set an alarm (A peaceful one!)
3. Take a Walk Outside
Getting some fresh air helps immensely to cleanse the mind and restore your willpower. When you go for a walk, your brain switches from thinking to observation mode. This allows it to rest and recover - making you feel refreshed. Again, 5-10 minutes walking around the block is enough to begin to restore your willpower and get back to work. 
Most of us believe that we have multiple stocks of willpower that we use to achieve different goals. We have one to resist cravings, one to make it to the gym and another to remain focused. Research has shown that this isn't the case. When you use willpower to make it through a boring meeting, you're going to have less left to clean when you get home.
This phenomenon can leave us to feel guilty about ourselves as we don't understand why we feel so tired. Simply recognizing that these feelings are due to low willpower and not a flaw in character can help you overcome them. If you recognize that you are low on willpower, try eating some fruit, taking 5-10 minutes to relax or go for a walk. You may just find that you have the willpower you need to take on the new challenge after all!
Strengthen Your Willpower
Get the willpower resources to overcome your biggest challenges
Baumeister, Roy F., and John Tierney. Willpower: Rediscovering the Greatest Human Strength. New York: Penguin, 2011.
Baumeister, Roy F., Ellen Bratslavsky, Mark Muraven, and Dianne M. Tice. "Ego Depletion: Is the Active Self a Limited Resource?" Journal of Personality and Social Psychology 74.5 (1998): 1252-265.
Pollan, Michael. Food Rules: An Eater's Manual. New York: Penguin, 2009. Print.
McGonigal, Kelly. The Willpower Instinct: How Self-control Works, Why It Matters, and What You Can Do to Get More of It. New York: Avery, 2012.