Have you ever wondered if skipping breakfast could make you feel foggy at work? Or if intermittent fasting might lead to irritability, distraction, and decreased productivity?
Snack food advertisements often stress that “you’re not you when you’re hungry”, reinforcing the widespread belief that regular eating is vital for maintaining cognitive sharpness.
This idea is deeply embedded in our culture, which often promotes continuous fueling as the key to alertness and efficiency.
However, time-restricted eating and intermittent fasting have emerged as popular wellness practices over the past ten years. Millions engage in these practices for various long-term benefits, including weight control and better metabolic health.
This leads us to an important question: can we enjoy the health benefits of fasting without jeopardizing our cognitive performance? To explore this, we undertook the most comprehensive review to date on the impact of fasting on cognitive abilities.
Why Fast in the First Place?
Fasting is more than just a trendy dietary approach; it leverages a biological system that has evolved over thousands of years to help humans manage periods of scarcity.
When we eat regularly, the brain primarily relies on glucose stored in the body as glycogen. However, after approximately 12 hours without food, these glycogen reserves start to deplete.
At this stage, the body performs an intelligent metabolic switch: it transitions to breaking down fat into ketone bodies, such as acetoacetate and beta-hydroxybutyrate, which serve as an alternative energy source.
This metabolic adaptability, once crucial for our ancestors’ survival, is now associated with a variety of health benefits.
Some of the significant benefits of fasting arise from its ability to transform bodily processes. For example, fasting activates autophagy, a cellular “cleanup crew” that removes damaged components and recycles them, a process believed to support healthier aging.
Additionally, fasting enhances insulin sensitivity, which allows the body to regulate blood sugar more effectively, reducing the risk of conditions such as type 2 diabetes.
Moreover, the metabolic changes initiated by fasting appear to provide broader protective benefits, lowering the risk of chronic diseases often linked to overeating.
The Findings
These physiological advantages have made fasting appealing to many. However, numerous individuals hesitate to try it, fearing that their mental capabilities will deteriorate without consistent food intake.
To investigate this concern, we conducted a meta-analysis, a review of existing experimental research that compared cognitive performance among individuals who were fasting and those who were fed.
Our search yielded 63 scientific articles, comprising 71 independent studies with a total of 3,484 participants assessed across 222 different cognitive measures. This research spanned almost seven decades, from 1958 to 2025.
Upon analyzing the data, our conclusion was unequivocal: there was no significant difference in cognitive performance between fasting and non-fasting healthy adults.
Participants performed equally well on cognitive tests assessing attention, memory, and executive functioning, whether or not they had eaten shortly before.
When Fasting Makes a Difference
Our analysis did uncover three crucial factors that can influence the impact of fasting on cognitive function.
First, age plays a significant role. Adults exhibited no notable decline in mental performance when fasting. However, children and adolescents tended to perform worse on tests when they skipped meals.
Their developing brains appear more sensitive to fluctuations in energy availability, which supports the longstanding recommendation that children should start their school day with a nutritious breakfast to enhance learning.
Timing also has an impact. We found that prolonged fasts were linked to a smaller performance gap between fasting and fed states. This may be due to the metabolic transition to ketones, which can maintain a consistent energy supply to the brain as glucose levels decline.
Cognitive performance for fasting individuals tended to be lower during later hours of the day, suggesting that fasting might exacerbate natural dips in our circadian rhythms.
The type of cognitive tasks also influenced the results. In tasks that involved neutral symbols or shapes, fasting participants performed equally well, or even slightly better.
However, when tasks included food-related cues, fasting participants tended to struggle. While hunger doesn’t universally cause cognitive fog, it can make us more susceptible to distractions related to food.
Practical Implications
For most healthy adults, these findings provide reassurance: you can experiment with intermittent fasting or other fasting methods without fearing a decline in mental clarity.
Nonetheless, fasting may not be suitable for everyone. Special care is advised for children and adolescents, whose brains are still developing and who may require regular meals for optimal performance.
Furthermore, if your job demands peak concentration during the later parts of the day or if you frequently encounter tempting food cues, fasting might present additional challenges.
Related: Study Raises Serious Questions About The Benefits of Intermittent Fasting
Lastly, individuals with specific medical conditions or dietary needs should consult with a professional before attempting fasting.
Ultimately, fasting should be viewed as a personal tool rather than a universal solution, with its benefits and challenges varying from person to person.
David Moreau, Associate Professor of Psychology, University of Auckland, Waipapa Taumata Rau
This article is republished from The Conversation under a Creative Commons license. Read the original article.
An earlier version of this article was published in November 2025.
