Mental fatigue weakens self-control and amplifies cravings for rewards like junk food and addictive substances.
Periods of stress and mental fatigue present significant challenges to maintaining self-discipline. A recent study published in PNAS Nexus reveals that after prolonged mental exertion, not only does self-control diminish, but the sensation of reward becomes more intense. By analyzing both human and animal behavior, the research sheds light on why self-regulation is more likely to falter under cognitive strain.
The connection between mental fatigue and lapses in self-discipline has long been recognized, particularly in high-stress situations. While previous studies primarily focused on a weakened ability to resist temptation, less attention had been given to whether fatigue also amplifies the appeal of rewards. By investigating this overlooked factor, new insights emerge on how stress contributes to unhealthy behaviors, including overeating and addiction, while offering potential strategies for enhancing self-regulation.
“I have spent many years doing research on self-control. It’s one of the most important human traits and a major key to success in life, personally, socially, and professionally.”
— Roy F. Baumeister, visiting professor at Harvard University and author of Willpower: Rediscovering the Greatest Human Strength.
Extensive research suggests that self-control functions like a muscle, becoming fatigued after exertion and performing less effectively. However, recent findings extend this understanding further.
Mental fatigue not only weakens self-regulation but also heightens emotions and cravings, creating a dual challenge—reduced resistance combined with intensified impulses.
“There is a ‘double whammy’—people have less ability to resist plus stronger impulses and desires.”
— Roy F. Baumeister
An unexpected collaboration led to these insights. During a conference presentation on research involving rats, a chance meeting between Marcello Solinas and Roy F. Baumeister revealed strikingly similar patterns in human studies. By combining data, compelling parallels emerged between cognitive exhaustion, addiction, and reward sensitivity.
“I met the lead author (Marcello Solinas) by accident at a conference, where he was presenting his research on rats. I told him we had similar findings with people. We decided to put our data together. To find the same effect with rats and cocaine, and with humans and other desires, is very exciting.”
— Roy F. Baumeister
Solinas, a CNRS director of research at the University of Poitiers, initially set out to examine whether cognitive engagement could reduce cocaine consumption. However, findings revealed the opposite effect, aligning with Baumeister’s ego-depletion theory.
“I was interested in the effects of cognitive exercise on addiction with the expectation that cognitive exercise would decrease cocaine intake. The results went in the opposite direction, which actually was a finding predicted by the ego-depletion theory proposed by Baumeister.”
— Marcello Solinas
The Study
Experiments with rats explored the impact of cognitive effort on reward-seeking behavior.
- One group underwent rigorous training in an attentional set-shifting task, requiring adaptability and focus to obtain food rewards.
- A second group followed a less demanding task where rewards were provided with minimal effort.
Afterward, all subjects were given access to either cocaine or saline. Some received a period of rest before exposure, while others had immediate access, allowing researchers to assess the role of recovery in mitigating impulsive behavior.
Cocaine consumption and locomotor activity, indicators of reward sensitivity, were measured in rats following cognitive exertion. Findings revealed that immediate access to cocaine led to higher consumption in the high-effort group compared to the low-effort group, suggesting that cognitive strain amplified the drug’s rewarding effects.
“The results in rats were completely surprising because we expected a decrease in cocaine intake after a cognitive exercise. In addition, the most parsimonious explanation of increased cocaine consumption in rats was not the usual depletion of self-control capacity but simply that cocaine was more rewarding after a cognitive effort.”
— Marcello Solinas
However, a period of rest before access reversed this effect, with high-effort rats consuming less cocaine than those in the low-effort group. Additionally, increased locomotor activity in the high-effort, no-rest group further supported the conclusion that cognitive exertion enhanced reward sensitivity.
The Human Experiments
A parallel study with human participants produced similar results in the context of food rewards. In one experiment, cognitive strain was induced by assigning participants to either a high-effort or low-effort mental task. The high-effort group was instructed to suppress thoughts of a “white bear” while completing a thought-listing exercise, a well-established method for inducing cognitive fatigue. Meanwhile, the low-effort group performed the task without any restrictions on thought processes. Following this, participants were given potato chips under the pretense of evaluating their taste and texture, with both consumption and reported enjoyment carefully measured.
Results showed that the high-effort group consumed more chips and rated them as more enjoyable than the low-effort group. Statistical analyses indicated that heightened enjoyment, rather than loss of self-control, was responsible for the increased consumption.
A second experiment sought to determine whether cognitive effort selectively enhanced the appeal of rewards or broadly influenced sensory perception. Participants engaged in a writing task with varying cognitive demands: the high-effort group composed essays while avoiding common letters (such as “A” and “N”), whereas the low-effort group avoided less frequently used letters (such as “X” and “Z”).
Following the task, a taste test involving chocolate was conducted alongside an evaluation of neutral objects, such as pens and sticky notes. Those in the high-effort group rated the chocolate as more enjoyable and expressed a greater desire for more, while evaluations of neutral items remained unchanged. These findings suggested that cognitive effort specifically amplified the appeal of rewards rather than broadly altering sensory perception.
The Studies Results
The overlap between human and rat data offers compelling insight into how fatigue influences reward sensitivity across species. Despite significant differences between the two, similarities in brain function suggest that experiencing intensified sensations when mentally exhausted is a deeply ingrained biological response.
“The convergence between the human and rat data is surprising to a scientist, though there are many similarities between human and rat brains. (But also huge differences, of course.) It means that the pattern of feeling things more strongly when fatigued is common across multiple species. It is not just human beings.”
— Roy F. Baumeister, PsyPost
These findings suggest that intense cognitive effort not only depletes self-control, as predicted by the ego-depletion theory, but also increases the desire for highly rewarding stimuli, such as addictive substances and calorie-dense foods.
This shift in reward sensitivity could contribute to impulsive behaviors with potential health consequences.
“Intense cognitive effort can not only reduce self-control (as predicted by the ego-depletion theory) but it can also increase the desire for strong rewards, drugs and palatable food, whose consumption could lead to unhealthy consequences. These are the immediate after-effects of cognitive effort. However, cognitive effort in the long run could be beneficial. The fact that this mechanism is conserved in rodents and humans suggests an evolutionary role of the increase of reward sensitivity induced by cognitive effort.”
— Marcello Solinas
While the study presents strong evidence linking mental fatigue to heightened reward sensitivity, some limitations remain. The rat experiments included only male subjects, limiting the ability to generalize findings across sexes. Additionally, differences in cognitive tasks between species make direct comparisons challenging. Cocaine served as a clear reward stimulus for rats, but ethical considerations required the use of food rewards in human studies, adding another variable to the analysis.
“One should always be careful, and future research might change the picture, though usually not completely. But we do have more studies with the same finding.”
— Roy F. Baumeister
Further research may explore variations in these effects across different demographics and reward types. Investigating the neural mechanisms behind these changes could clarify how mental fatigue alters activity in brain regions associated with reward processing, such as the prefrontal cortex and nucleus accumbens. This understanding could lead to new strategies for managing addiction and improving self-regulation.
“We would like to understand the neurobiological mechanisms underlying these effects, which is possible in rats, in order to design novel strategies to reduce the negative effects of cognitive effort.”
— Marcello Solinas
“Understanding self-control is a key to success in life, also a key to understanding the self. For me, the big picture is to understand the human self. My understanding after almost a half a century of research is covered in my 2022 book, The Self Explained: How And Why We Become Who We Are.”
— Roy F. Baumeister
https://doi.org/10.1093/pnasnexus/pgae432
Abstract
An important body of literature suggests that exerting intense cognitive effort causes mental fatigue and can lead to unhealthy behaviors such as indulging in high-calorie food and taking drugs. Whereas this effect has been mostly explained in terms of weakening cognitive control, cognitive effort may also bias behavioral choices by amplifying the hedonic and emotional impact of rewards. We report parallel findings with animals and humans supporting this hypothesis. In rats, exerting cognitive effort immediately before access to cocaine self-administration significantly increased drug intake. In addition, exerting cognitive effort increased the psychostimulant effect of cocaine. The effects of cognitive effort on addiction-related behaviors were eliminated and even reversed when animals could rest in their home-cage for 2–4 h before access to cocaine self-administration. Among humans, we found that expending cognitive effort increased consumption of tasty (but unhealthy) food by increasing the hedonic enjoyment of consuming the food. In addition, the effects were specific for emotionally relevant stimuli (i.e. food rewards) and did not generalize to judgment about neutral objects. Altogether these data suggest that intense cognitive effort can increase the perceived intensity of rewards and lead to their overconsumption. This effect may contribute to bad decision making induced by excessive cognitive effort and make people more vulnerable to indulge in unhealthy behaviors such as use of addictive drugs.