Does 16:8 Fasting Improve Focus and Memory? A Trial Says No

A small randomized trial from New Zealand tested whether four weeks of 16:8 fasting changes working memory or attentional control in healthy women. It found no difference between the fasting group and the control group, in either direction. Fasting did not sharpen thinking, and it did not dull it.

That is a null result, and it is worth reading closely, because both "fasting clears your mind" and "fasting gives you brain fog" are common claims.

What the study did

The paper was published online on 28 July 2026 in Acta Psychologica by researchers in the Department of Psychology at the University of Otago. The authors call it a randomized controlled pilot.

Healthy adult women were randomly assigned to one of two groups for four weeks. One group followed a 16:8 time-restricted eating pattern: 16 hours of fasting and an 8-hour eating window each day. The other group kept eating as usual.

Every week, both groups completed the same set of computer tasks. One of them, the Symmetry Span task, measures working memory, the ability to hold information in mind while doing something else. Three others (Stroop Squared, Simon Squared and Flanker Squared) measure attentional control, the ability to stay on the relevant signal and ignore the distracting one. The researchers also tracked well-being, body weight and reported adverse events.

Who	Healthy adult women
Groups	Fasting (23 participants) and Control (20 participants)
Fasting pattern	16:8, daily, for 4 weeks
Comparison	Usual diet
Tested weekly	Working memory, attentional control, well-being, body weight
Cognitive result	Both groups improved over time, no significant difference between them
Other results	Significant BMI reduction in the fasting group compared with controls, no group difference in reported adverse events

What it found

Scores on working memory and attentional control went up over the four weeks. They went up in both groups, and the abstract reports no significant differences between them.

The abstract does not say why both groups improved. One ordinary explanation is practice: when people repeat the same tasks every week, they tend to get better at them. This is exactly why the control group matters. Without it, a rise in scores among people who were fasting would have looked like an effect of fasting.

Two other results are reported. The fasting group showed a statistically significant reduction in BMI compared with the control group; the abstract does not give the size of that change, so we will not guess at one. And the two groups did not differ in reported adverse events.

The authors' summary is careful: short-term 16:8 eating was feasible and well tolerated in this pilot sample, was associated with reduced BMI, and showed "no evidence of TRE-specific improvement or impairment in the tested cognitive functions."

What a four-week pilot can and cannot rule out

A null result from a small study is not the same as proof that there is no effect. It helps to be exact about what this one can bear.

What it makes less likely:

• A large, fast improvement in working memory or attention from 16:8 in healthy women. If the effect were big, a randomized study with weekly testing would have had a reasonable chance of seeing some sign of it.
• A large impairment. If skipping breakfast or eating in an 8-hour window clearly damaged concentration on tasks like these, that should have shown up too.

What it cannot rule out:

• Small effects. With 23 and 20 participants, the study can only detect fairly large differences. A modest benefit or a modest cost could be hidden in the noise. That is the main limit of a pilot, and the authors use that word themselves.
• Longer-term effects. Four weeks is short. The trial says nothing about what happens after several months.
• Other people. The participants were healthy women. The result does not automatically apply to men, to older adults, or to people with a medical condition.
• Other kinds of thinking. Four tasks were used. Long-term memory, mood-dependent concentration during a working day and creative work were not what these tasks measure.

So the fair reading is narrow: in this pilot, 16:8 for four weeks neither helped nor harmed the cognitive functions that were tested.

What this means if you fast

If you chose 16:8 hoping for better focus, this study gives you no reason to expect it. If you avoided 16:8 because you worried it would wreck your concentration at work, this study gives no support for that worry either, at least in healthy women over four weeks.

That leaves your own experience as the best guide. A few practical points follow from the design of the study rather than from any promise of a result:

• Judge it over weeks, not days. The researchers tested weekly for a month. One foggy morning in the first week tells you little.
• Compare like with like. Scores rose in the control group too. If you feel more focused after a month, ask what else changed: sleep, workload, caffeine, a holiday.
• Pick the schedule for reasons that hold up. A plan that fits your work and family meals is easier to keep than one chosen for a benefit that has not been shown. The 16:8 schedule page explains the pattern, and how to choose the right fasting schedule for your lifestyle compares it with the alternatives.
• If you do feel flat in the afternoon, look at what and when you eat inside the window before blaming the fast. How to structure your eating window for sustained energy covers that.

This is general information, not medical advice. If you are pregnant or breastfeeding, have diabetes or take blood-sugar medication, have a history of eating disorders, take other regular medication, or are under 18, talk to a doctor before fasting.

QoFast is a fasting tracker app for iPhone and Android with intermittent plans including 16:8, 18:6, 20:4, OMAD and custom windows. It also has mood logging and a fasting history, which is a simple way to keep your own notes on how you feel across several weeks. QoFast gives general information, not medical advice, and it is free to download with an optional Pro subscription: App Store and Google Play.

FAQ

Does intermittent fasting improve concentration? This trial did not find that. Over four weeks of 16:8, working memory and attentional control improved equally in the fasting group and in the group eating as usual.

Does 16:8 fasting cause brain fog? The trial found no evidence of impairment on the tasks it used. It was small and short, so it cannot exclude a small effect or an effect in particular individuals.

Why did the control group also get better? The abstract does not give a reason. Repeating the same tests every week usually leads to better scores through practice, which is why a control group is needed to interpret the result.

Did the fasting group lose weight? The abstract reports a statistically significant reduction in BMI in the fasting group compared with controls. It does not state how large the reduction was.

Sources

• McLeod SC, Surrey T, McCormack JC, Conner TS, Peng M. "Impact of a 4-week time-restricted eating (16:8) intervention on working memory and attentional control in adult women: A randomized controlled pilot." Acta Psychologica, published online 28 July 2026. DOI: 10.1016/j.actpsy.2026.107510, PubMed 42520714