What do the following acts have in
common?

  • choosing the healthy items at the breakfast
    buffet and ignoring the pastries and fried food
  • smiling and saying, “Let’s see if I can help
    you with that,” when an angry client blows a minor issue into a major complaint
  • sticking to a budget, even after your initial
    enthusiasm for improving your finances has waned, and the sale at your favorite
    store is beckoning
  • staying on the treadmill for the time goal you
    set, even though each minute is a little harder than the last

Each of these acts requires willpower—the ability to ignore
temporary pleasure or discomfort to pursue a longer-term goal. It’s easy to
agree that these challenges require inner strength, but where does that
strength come from? And why is willpower such a fickle friend, supporting us on
some occasions and abandoning us on others?

Fifteen years ago, Roy F. Baumeister, PhD, now a researcher at
Florida State University, set out to answer these questions. He pitted several
possible models of willpower against each other, and when the dust of his early
studies settled, the results supported a surprising model. According to
Baumeister, willpower is not a personality trait, a skill or a virtue. Instead,
it operates like a muscle. And as such, it can be strengthened—but also easily
exhausted (Baumeister 2003).

This “strength” model of willpower has important implications for
fitness and wellness professionals who seek to inspire and support healthy
behavior in others. By understanding how willpower can be strengthened, you can
find new strategies for helping clients meet their goals. And by understanding
why willpower is necessarily limited, you can identify ways of supporting
behavior change without exhausting willpower.

A strength model of willpower proposes four important ideas:

  1. Willpower is a mind-body response, not merely a
    mindset.
  2. Using willpower depletes resources in the body.
  3. Willpower is limited.
  4. Willpower is trainable.

Let’s consider each of these ideas; the evidence that supports
them; and how they can be applied to health behaviors.

Willpower Is in the Mind and Body

Mind-body
responses are coordinated physiological changes that allow you to adapt to some
challenge. The best-known mind-body response is the fight-or-flight response to
stress or danger—heart racing, blood pressure soaring, muscles tightening and
senses heightened (Sapolsky 2004). The relaxation response, in contrast, allows
the body to respond to its internal needs of digestion, growth and restoration
(Benson 1975).

Suzanne Segerstrom, PhD, a psychologist at the University of
Kentucky, was one of the first researchers to study the biological basis of
willpower. Segerstrom’s research has begun to identify changes in the
autonomic, cardiovascular, neuroendocrine and immune systems during acts of
willpower. Segerstrom says it’s possible that these changes are part of a
coordinated whole-body response that helps us adapt to challenges requiring
self-control. She calls it a “pause and plan” response. This mind-body response
would allow us to temporarily freeze our impulses and focus on our long-term
goals.

What’s going on in the body that helps us slow down and proceed
with intention, not instinct? One important process is heart rate variability
(HRV), or the beat-to-beat fluctuations in heart rate. In one study, Segerstrom
required participants to flex their willpower by eating carrots and resisting
cookies (Segerstrom & Solberg Nes 2007). Successful participants showed a
temporary increase in HRV as they were exerting their willpower, followed by a
drop afterward. What’s more, participants who had higher HRV when they joined
the study showed greater willpower.

Why would higher HRV assist self-control? Changes in HRV are
determined by the autonomic nervous system’s balance of sympathetic activation
(responsible for the fight-or-flight response) and parasympathetic activation
(responsible for the relaxation response). High HRV reflects a healthy,
flexible activation of both systems (Appelhans & Luecken 2006). Maintaining
high HRV in the face of a self-control challenge might represent a kind of
inner strength and stress resilience—the ability to focus attention (as in the
fight-or-flight response) but also to stay calm (as in the relaxation
response). The result: acting in line with our highest goals, and not with our
immediate appetites.

Willpower Depletes Resources in the Body

The mind-body response of exerting
willpower literally fatigues us (Tice et al. 2007). It depletes physical power,
as shown in one study that looked at the effects of mental self-control on
physical stamina (Bray et al. 2008). In this study, trying to control one’s
thoughts decreased muscular endurance, as measured by performance and EMG
activity. The researchers who conducted this study called the effect “central
fatigue.”

Our minds and bodies draw from the same source of strength. But
what is this source? Matthew Gailliot, PhD, a researcher at the Univer­sity of
Amsterdam, argues that the fuel of willpower is glucose. In a way, this should
be no surprise, since glucose is the primary fuel of the body. It’s the same
fuel that skeletal muscles use to exert their strength, and the same fuel that
the brain uses to perform its many tasks. What is surprising is the fact that
willpower uses a lot of it, even when the body is at rest. According to
Gailliot, willpower is a particularly expensive mental act, engaging many areas
of the brain and requiring high levels of fuel (Gailliot et al. 2007).

Gailliot and his colleagues have demonstrated that simple acts of
self-control both require and lower blood glucose levels. In one recent set of
studies (Gailliot et al. 2007), participants were required to control their
thoughts, emotions or behavior. Immediately following each act of self-control,
participants’ blood glucose levels dropped. Levels did not drop dur­-ing
similar tasks in which participants were not exerting self-control.
Participants were then asked to repeat the acts of self-control. Those whose
blood glucose levels had dropped the most from the first task performed the
worst on the second attempt. Participants given a sugary drink between tasks
(replenishing their blood glucose levels) were better able to exert their
willpower in the second attempt.

This study certainly pushes the boundaries of how we think about
willpower. Not many of us would like to believe that our virtue is dependent on
the last time we snacked. However, the implications are important: our ability
to flex our willpower depends on the availability of blood glucose. When blood glucose
levels are low (for example, from skipping breakfast), or when the body has
impaired use of glucose (as in diabetes or the increasingly common metabolic
syndrome), willpower is impaired. Numerous studies confirm that low blood
glucose levels and poor glucose use are associated with worse self-control of
attention, emotions and behavior (Gailliot et al. 2007).

Does this mean we should be giving ourselves sugar every time we
want to exert some self-control? Clearly, that would be a short-sighted strategy.
In their report of this research, Gailliot and his colleagues recommend a more
reasonable strategy: choose foods that keep blood glucose levels relatively
stable, and don’t skip meals.

The Limits of Willpower

The studies described above point to a troubling
idea: willpower is inherently limited. No matter how physically fit we are,
exerting ourselves inevitably leads to exhaustion. The same is true of inner
strength: self-control depletes willpower in much the same way that exercise
temporarily depletes physical power.

This observation comes from numerous studies over the last decade
that test the limits of people’s self-control (for a research review, see
Baumeister 2003). In these experiments, people who exerted their willpower on
one occasion struggled to do so a second time. Studies have found this effect
with all sorts of self-control tasks, including turning down tempting foods,
suppressing emotions and persevering at challenging problems. The conclusion:
the willpower “muscle” gets fatigued.

Taking the strength model literally, fitness pros might be
curious: Do we have separate willpower “muscles” for different types of
self-control? Does it take a different kind of strength to resist gossiping
than it does to get up early to exercise? Or do all acts of self-control draw
on the same source of strength?

According to willpower researcher Suzanne Segerstrom, PhD,
“Willpower is general. People may differ in how tempting they find chocolate
cake or slouching, but you don’t have separate will ‘powers’ for refusing
chocolate cake and for sitting up straight.” Research backs this up—the same
studies that show how willpower is limited also show it doesn’t matter whether
you control your temper or your craving for a cigarette—exerting willpower in
one area limits your ability to do so in all others.

Because willpower is limited, each act of self-control is a
win-lose effort, helping in the immediate situation but making us more likely
to lose control later. Refraining from gossiping at work makes it more difficult
to hit the gym after
work. Resisting the impulse to splurge at your favorite store makes it more
difficult to turn down dessert.

This means that it’s important to set priorities and to give
ourselves a break on the things that aren’t at the top of the list. Fitness and
health coach Chris Freytag, a member of the American Council on Exercise board
of directors, says this strategy is especially important in the United States,
where we receive the message that it’s possible to have it all and do it all.
According to Freytag, this message gets in the way of real change. “It’s the
whole American attitude of all or nothing. We have no middle ground. If you
change everything, it’s really hard to comply—it’s too much stress in your
life. You can only control so many things.” Freytag counsels people to aim for
progress, not perfection.

Willpower is also affected by whatever is going on in our lives.
Coping with any kind of stress depletes willpower (Gailliot et al. 2007). For
example, Kathleen Vohs, PhD, formerly a researcher at the University of British
Columbia and now assistant professor of marketing at the University of
Minnesota, has shown that having to control our image can deplete willpower
(Vohs, Baumeister & Ciarocco 2005). Managing our self-presentation in
professional or social settings—especially when repressing our natural
personality—takes a toll on our ability to meet other goals.

Social stress takes a particularly large toll on willpower. One
set of studies has shown that people who are lonely or socially rejected have
worse control over their thoughts, emotions and health-enhancing behaviors
(Baumeister et al. 2005). For example, socially rejected participants were less
likely to resist the temptation of freshly baked cookies, and they quit a
challenging task sooner than participants who were not rejected. Research also
shows that feeling stigmatized depletes willpower for both mental and physical
acts of self-control (Inzlicht, McKay & Aronson 2006).

Why might loneliness or feeling stigmatized—whether because of
race, gender or weight—keep us from our goals? Brain-imaging studies show that
social rejection impairs areas of the brain that are important for focusing
attention and pursuing goals, leading to worse self-control (Campbell et al.
2006). Other researchers have argued that social rejection takes away the will
to exert willpower, draining motivation as much as strength (Baumeister et al.
2005).

Aimee Kimball, PhD, director of mental training at the University
of Pittsburgh Medical Center’s (UPMC) Center for Sports Medicine, says this
research confirms what she has observed when motivating sedentary adults to
become active through UPMC’s START program. “Social support is really important
for developing willpower,” says Kimball. She encourages people to share their
goals and celebrate their successes with friends, co-workers or family. She
also counsels clients to simplify other things in their life. “I ask them to
write a list of things that stress them out, and make a plan for the things
they can control. If there’s something on the list they can’t control, I ask
them to cross it off so they don’t waste energy on it.”

Restoring Willpower

Is there anything we can do to restore
willpower once we’ve exhausted our strength? One strategy seems obvious: get
adequate physical rest. And, in fact, research suggests that sleep does restore
depleted willpower (Baumeister 2003). This is particularly important to keep in
mind when asking clients to make a challenging behavior change. Trying to do so
without enough sleep is like trying to drive a car without gasoline.

Michael Krugman, founder and director of the Sounder Sleep
System, agrees that addressing insomnia and insufficient sleep is a crucial
component of any health program. “When we’re sleep deprived, our resolve
weakens and we seek the path of least resistance. We spontaneously revert to
unhealthy habits that can be performed automatically, with less mental and
physical effort.” A good example of this is diet: people who sleep only 4 hours
a night are less likely to choose foods based on health, weight or ethical
concerns (Wells & Cruess 2006). If you need the strength to take the more
difficult path, you need proper rest.

Research also shows that a small boost in mood restores willpower
depleted by a recent act of self-control (Tice et al. 2007). For example, in
one study, participants’ willpower was depleted by the experimental equivalent
of dieting: they were left alone in a room with radishes, cookies and
chocolate, and were encouraged to help themselves—but only to the radishes!
Participants were then asked to perform a challenging mental task. Participants
who watched a short comedy video between the two tests of willpower persevered
longer in the challenging task.

In another study by the same researchers, participants who
received a gift from the experimenters (a small bag of candy tied up in a
ribbon) had better willpower for consuming a “health” drink that tasted like
vinegar. Why did a spoonful of sugar make the medicine go down? The researchers
argued that the happy surprise re-energized depleted stores of willpower (Tice
et al. 2007). It’s a well-known strategy to reward ourselves after we’ve followed through
with a new healthy behavior. But it might be an even better idea to reward
ourselves first, and use the mood boost to follow through.

These strategies—get enough sleep, laugh, focus on the positive,
and treat yourself—may not be new, but even the best advice needs to be
packaged in an appealing way. Thinking of these strategies as little “booster
shots” of willpower can motivate your clients to follow them more consistently
and can give people a sense of greater control over their willpower reserves.

Increasing Willpower

If willpower is like a muscle, then it
should be trainable through “exercise.” According to the physiology of fitness,
temporarily exhausting a muscle should lead to increased muscle size or
improved ability to use fuel. Is the same true for willpower?

Researchers have put this to the test with willpower training
regimes (Baumeister et al. 2006). These interventions take a simple approach:
ask people to control one thing that they aren’t used to controlling, and to do
it every day. Like physical exercise, this act of self-control can be
uncomfortable at first, but over time it is no longer a struggle. The willpower
muscle has learned a new skill, and with practice, the act of self-control is
less likely to deplete willpower.

If all acts of willpower reflect a single strength, then training
any individual act of self-control should strengthen all acts of self-control. Indeed, this is what
research shows. Committing to small, consistent acts of willpower in any
domain—from improving our posture to watching our finances—can increase overall
willpower (Muraven et al. 1999; Oaten & Cheng 2007).

For example, Megan Oaten, PhD, and Ken Cheng, PhD, researchers at
Macquarie University in Sydney, Australia, investigated the benefits of an
8-week willpower training program that required participants to create and meet
artificial deadlines (Oaten & Cheng 2006a). Participants not only improved
at time management; they also ate a healthier diet, increased their physical
activity levels and reduced tobacco, alcohol and caffeine use. Importantly,
these health behaviors were not addressed in the intervention. The primary
training effect appears to have been improved willpower endurance. Researchers
Oaten and Cheng argue that the training made participants less susceptible to
willpower depletion and therefore better able to take care of themselves.

We’ve been talking about “exercising” willpower, meaning exerting
it in any way. But what about actual physical exercise? Although it’s sometimes
hard for fitness professionals to imagine, the great majority of people find
exercising unpleasant and difficult. So showing up for a workout—and sticking
with it even when tired or bored—requires willpower. And according to the
strength model of willpower, exercise should also temporarily exhaust willpower
reserves. Over the long-term, however, it should lead to increased strength or
endurance of the willpower “muscle.”

This prediction is supported by recent studies examining the
effects of physical exercise on willpower. In a study by Oaten and Cheng
(2006b), participants were given free gym membership and personalized training
programs. After 2 months, participants who kept up with the exercise program
also reported improvements in a wide variety of behaviors requiring willpower,
from eating a healthier diet to controlling their temper to reducing substance
use and impulsive spending.

But what if you like to exercise? Will working out still improve
your willpower? Consider this: regular physical exercise can increase heart
rate variability (Buch, Coote & Townend 2002) and improve blood glucose
control (Snowling & Hopkins 2006). The form of the exercise doesn’t seem to
matter, as long as it’s consistent. Recent studies suggest that mind-body
exercise, such as yoga, is also effective (Khattab et al. 2007, Shapiro et al.
2007).

Willpower researcher Matthew Gailliot, PhD, agrees that physical
exercise could improve willpower by enhancing HRV and blood glucose control.
However, no research has directly tested this possibility. Researcher Suzanne
Segerstrom, PhD, advises, “Until we understand more about it, exercise might be
the best bet for improving willpower, because it covers both bases.” Add that
to your list of reasons everyone should be physically active!

Conserving and Bypassing Willpower

If willpower is
inherently limited, it makes sense to conserve it. But it doesn’t make sense to
simply give in to temptation right and left. How can you make healthy choices
without depleting willpower?

Margaret Moore, founder and chief executive officer of
Wellcoaches® Corporation, teaches her clients to plan in advance as a strategy
for conserving willpower. “You don’t want to be standing in front of the fridge
saying you don’t have a clue what to eat. The weakest moment of self-control is
when you’re hungry and tired. Too much choice tends to overwhelm us. If you have
to make a lot of choices, you’ll deplete your self-control.” This advice is
backed up by recent studies showing that any type of decision making—even
choosing among brands of candy or shampoo—can deplete willpower (Vohs et al.
2008).

Moore recommends organizing your life so you don’t have to think
about what you’re going to eat or whether you’re going to exercise. Make
healthy choices in advance, and in moments of greatest strength. This is a
strategy you can share with clients. For example, plan your dinner before you
come home hungry and exhausted from work. Set up a weekly date to meet a friend
at the gym, and don’t consult your temptation to stay home. Commit and just
show up—no choice and no willpower needed.

Another strategy is figuring out how you will deal with
obstacles. One study found that people who wrote in a journal about how they
would handle barriers to exercise were more likely to stick with an exercise
program (Sniehotta et al. 2005). A second study found that this approach helped
people succeed at a challenging self-control task after a previous task had
depleted their willpower (Webb & Sheeran 2003).

Sports mental health trainer Aimee Kimball, PhD, says this is an
important aspect of preparing people to start and stick with an exercise
program. “I ask them to identify obstacles to their goals, so they can make a
plan.” Kimball also asks clients to visualize themselves following through with
their plans, so that success seems not only possible but inevitable.

This is a process you can apply to any self-control challenge.
Deciding what you will do when tempted sets up a kind of reflex response. You
don’t have to think about what you will do—you just use the challenge as a cue
to implement your planned strategy. For example, you might decide in advance
that every time you are tempted to eat before bed, you will take a hot bath
instead. And then when you get the craving, it’s a signal to take a bath, not
hit the fridge.

Finally, it may be possible to face a challenge by completely
bypassing the willpower system and drawing on a different source of strength:
motivation. The best way to do this is to focus on the positive reward of a
behavior rather than the sacrifice or effort. Health coach Margaret Moore takes
a big-picture approach to positive motivation: “I ask clients to ask
themselves, ‘Who do I want to be? Who is my best self when it comes to health
and well-being?’ People need to have some self-image in mind. I also ask them
to figure out ‘Why do I care? What’s the higher purpose? What will being
healthy give me that my life will not be great without?’ That’s the heartfelt
piece that people don’t typically connect to. If people don’t get moved
emotionally, if they don’t know what they want, then they’re not going to
succeed.”

Limits and Possibilities

The idea that willpower is inherently
limited is a tough idea for many people to accept. We’d rather believe that our
own inner strength is limitless and that if we just try hard enough, we can
kick any bad habit, stick to any diet and follow any fitness program, while
simultaneously winning friends and influencing people. It’s also tempting to
look at those who don’t succeed at a goal and judge them as being unwilling or
weak. But if everything from stress to blood sugar levels can get in the way of
willpower, it’s no wonder we aren’t all self-control superheroes.

This shouldn’t be taken as bad news, and it doesn’t mean we’re
all doomed to self-control failure. A strength model of willpower gives you and
your clients a realistic and creative way to plan for success. Among the top
lessons of the research are the following:

  • Willpower is limited, so it’s important to set
    reasonable goals and priorities. Conserve your willpower for what really
    matters.
  • Forgive temporary setbacks. A single mistake
    doesn’t mean you are weak. It may just mean you’ve already succeeded to the
    limits of your current ability, and now you deserve a rest or reward to restore
    your strength.
  • Recognize that willpower is not “all in the
    mind,” and supply your mind-body with the fuel it needs to face life’s
    challenges. This fuel includes rest, a healthy diet and a steady supply of
    positive experiences.
  • Understand how the demands of your job, family
    and other relationships may interfere with your (and your clients’) ability to
    stick with a health or fitness program. Look for ways to reduce stress in all
    areas of your life to support any major life change.
  • Conserve or bypass willpower by focusing on
    other strengths: planning, commitment and positive motivation.

SIDEBAR: Why Diets Fail

Think
of the typical diet: you’re required to repeatedly avoid foods you love and
show incredible self-restraint around other people who aren’t dieting. Who has
the strength to exert unrelenting willpower, especially when life presents so
many other challenges that require self-control? Research shows that chronic
dieters are especially susceptible to overeating when faced with an abundance
of tempting food, specifically because they view eating as a constant act of
self-control (Vohs & Heatherton 2000).

Traci Mann, PhD, associate professor at the University of Minnesota
who studies dieting and other forms of self-control, says, “The majority of
people will not be able to succeed at a severely restrictive diet because it is
simply too hard. It’s just unsustainable.” Mann recommends that people make
smaller but meaningful changes in their diets, such as cutting out just one
type of unhealthy food or reducing calorie intake by only 200 calories a day.
“These types of changes are not as painful and don’t require as much
self-control, so people may be able to sustain them longer.”

Giving yourself breaks from willpower—for example, taking 1 day off a
week to indulge in your favorite foods—might also replenish your resources to
resist temptation. Health and fitness coach Chris Freytag, author of Prevention’s Shortcuts to
Big Weight Loss
(Rodale 2007), says, “You have to give yourself a
little satisfaction every now and then. If you deprive yourself all the time,
something’s going to give. You have to pick your battles.”

Kelly
McGonigal, PhD, teaches psychology, yoga and group fitness at Stanford
University. Contact her at [email protected] or www.openmindbody.com.

References

Appelhans,
B.M., & Luecken, L.J. 2006. Heart rate variability as an index of regulated
emotional responding.
Review
of General Psychology, 10
(3), 229–40.

Baumeister,
R.F. 2003. Ego depletion and self-regulation failure: A resource model of
self-control.
Alcoholism:
Clinical and Experimental Research, 27
(2), 281–84.

Baumeister,
R.F., et al. 2005. Social exclusion impairs self-regulation.
Journal of
Personality and Social Psychology, 88
(4), 589–604.

Baumeister,
R.F., et al. 2006. Self-regulation and personality: How interventions increase
regulatory success, and how depletion moderates the effects of traits on
behavior.
Journal
of Personality, 74
(6), 1773–1802.

Benson, H.
1975.
The
Relaxation Response.
New York: Morrow.

Bray, S.R.
et al. 2008. Effects of self-regulatory strength depletion on muscular
performance and EMG activation.
Psychophysiology, 45, 337–43.

Buch, A.N.,
Coote, J.H., & Townend, J.N. 2002. Mortality, cardiac vagal control and
physical training: What’s the link?
Experimental Physiology, 87
(4), 423–35.

Campbell,
W.K., et al. 2006. A magnetoencephalography investigation of neural correlates
for social exclusion and self-control.
Social Neuroscience, 1 (2),
124–34.

Gailliot,
M.T., et al. 2007. Self-control relies on glucose as a limited energy source:
Willpower is more than a metaphor.
Journal of Personality and Social
Psychology, 92
(2), 325–36.

Inzlicht,
M., McKay, L., & Aronson, J. 2006. Stigma as ego depletion: How being the
target of prejudice affects self-control.
Psychological Science, 17
(3), 262–69.

Khattab, K.,
et al. 2007. Iyengar yoga increases cardiac parasympathetic nervous modulation
among healthy yoga practitioners.
Evidence-based Complementary and
Alternative Medicine, 4
(4), 511–17.

Mann, T., et
al. 2007. Medicare’s search for effective obesity treatments: Diets are not the
answer.
American
Psychologist, 62
(3), 220–33.

Muraven, M.,
Baumeister, R.F., Tice, D.M. 1999. Longitudinal improvement of self-regulation
through practice: Building self-control strength through repeated exercise.
Journal of Social
Psychology, 139
(4), 446–57.

Oaten, M.,
& Cheng, K. 2006a. Improved self-control: The benefits of a regular program
of academic study.
Basic
and Applied Social Psychology, 28
(1), 1–16.

Oaten, M.,
& Cheng, K. 2006b. Longitudinal gains in self-regulation from regular
physical exercise.
British
Journal of Health Psychology, 11
(4), 717–33.

Oaten, M.,
& Cheng, K. 2007. Improvements in self-control from financial monitoring.
Journal of Economic
Psychology, 28,
487–501.

Sapolsky, R.
2004.
Why
Zebras Don’t Get Ulcers: An Updated Guide to Stress, Stress-Related Diseases,
and Coping.
New York: Holt.

Segerstrom,
S.C., & Solberg Nes, L. 2007. Heart rate variability reflects
self-regulatory strength, effort, and fatigue.
Psychological Science, 18
(3), 275–81.

Shapiro, D.,
et al. 2007. Yoga as a complementary treatment of depression: Traits and moods
on treatment outcome.
Evidence-based
Complementary and Alternative Medicine, 4
(4), 493–502.

Sniehotta,
F.F., et al. 2005. Long-term effects of two psychological interventions on
physical exercise and self-regulation following coronary rehabilitation.
International
Journal of Behavioral Medicine, 12
(4), 244–55.

Snowling,
N.J., & Hopkins, W.G. 2006. Effects of different modes of exercise training
on glucose control and risk factors for complications in type 2 diabetic patients:
A meta-analysis.
Diabetes
Care, 29
(11), 2518–27.

Tice, D.M.,
et al. 2007. Restoring the self: Positive affect helps improve self-regulation
following ego depletion.
Journal of Experimental Social Psychology, 43,
379–84.

Vohs, K.D.,
Baumeister, R.F., & Ciarocco, N.J. 2005. Self-regulation and
self-presentation: Regulatory resource depletion impairs impression management
and effortful self-presentation depletes regulatory resources.
Journal of
Personality and Social Psychology, 88
(4), 632–57.

Vohs, K.D.,
et al. (in press/May 2008). Making choices impairs subsequent self-control: A
limited resource account of decision making, self-regulation, and active
initiative.
Journal
of Personality and Social Psychology.

Vohs, K.D.,
& Heatherton, T. F. 2000. Self-regulatory failure: A resource depletion
approach.
Psychological
Science, 11
(3), 249–54.

Webb, T.L.,
& Sheeran, P. 2003. Can implementation intentions help to overcome
ego-depletion?
Journal
of Experimental Psychology, 39,
279–86.

Wells, T.,
& Cruess, D.G. 2006. Effects of partial sleep deprivation on food
consumption and food choice.
Psychology and Health, 21, 79–86.