The Essential Guide to Your Brain and Fasting

Our cognitive functioning is extremely important for us. Judging from an evolutionary and developmental perspective, it has allowed us to become the species that now dominates Earth. Especially higher cognitive functioning is something people have tried to improve through different methods.

Nootropics are probably one of the more known methods. But that’s not what I’m going to talk about here. The main issue to be presented here is fasting. I’ve written about fasting in the past, intermittent fasting to be more precise. In that post, I’ve laid out all the physical benefits that go together with intermittent fasting. When we fast, we subject ourselves to caloric restriction – we’re getting enough macro- and microutrients, but are achieving a caloric deficit. That means we’re eating fewer calorie than we are spending. During this bodily state, some interesting things start to happen. Studies have shown many positive health benefits, at least when it comes to our physical health.

In general, the internet is saturated with content on the positive physical health benefits of intermittent fasting and caloric restriction. However, very little has been written about the cognitive benefits and shortcomings that occur when we commit to fasting.

This post intends on changing this.

There are many reasons why someone would decide to voluntarily skip a meal. The reason can be religious, it can be part of a weight loss program, it can be a consequence of a physical or mental illness (think anorexia), or it can be simply a result of a busy schedule.

But when talking about the effects of fasting on people, we have to distinguish between short-term and long-term fasting. The latter usually lasts for a few days, while the former lasts only for a day or a few hours. And this is important, because long-term fasting; achieving a caloric deficit for prolonged periods of time, leads our bodies into a so-called starvation response. It’s commonly called; and sometimes sadly misunderstood by the fitness community, as starvation mode.

The presented effects will be based on different cognitive domains and the duration of fasting that was undertaken.


Psychomotor abilities

Psychomotor abilities are abilities/skills that influence your capacity to manipulate and control objects. Hand-eye coordination, balancing, and reaction times in different tasks are some of the things scientists are usually interested when they are measuring these things. But because of a multitude of abilities that are involved here, it is hard for researchers to study them in a uniform and standardized way. Up to this point, they have measured decreases and increases in performance by different means.

Some have been testing the number of taps people make as well as their reaction times:

If you were to fast for over 24h and were told to tap one or two fingers, you would probably do worse compared to people who didn’t fast – the number of your taps would decrease.  [1, 2]

However, if the fast was a short one, where you would skip one or two meals during a day, there would probably be no difference in the number of your taps or your speed in general. [2, 3]

Another prevalent way of testing psychomotor abilities during fasting includes reaction times.

This was/is being measured in two different ways:

  • People have to press a button after a certain stimulus is shown to them
  • People have to press different buttons when different stimuli are presented to them.

If you are fasted, your reaction time shouldn’t increase under the first condition – so you should be equally fast to press the button when a certain stimulus is shown to you [2, 4], Your reaction time should improve under the second condition. So you should be faster to react when fasted if you get to choose which button to press after a certain stimulus [5]. In one study, however, Muslims during Ramadan needed a longer time to respond; their reaction times deteriorated. But this became clear only after six days of fasting. It’s quite possible that the increases in how long they needed to respond were a consequence of fatigue, as it was their sixth day of the fast [6]. It’s hard to determine the real cause and effect.

With reaction times and tapping behind us, let’s move to memory.



I don’t think I need to explain in details what memory is. Maybe worth explaining is the concept of working memory. Your working memory dictates how many pieces of information can you keep in your mind at a given time. For most people it’s 7 pieces of information, give or take two. There’s a lot more to be written about it, but this should suffice for now.

Different studies on memory so far have shown:

  • If you are fasting, your ability to memorize words and repeat them afterwards won’t change. [1, 2, 7, 8, 9]
  • It is possible that you would do better if you ate breakfast before being tested for this. However, the evidence is very shaky here.

The results of one study are very doubtful in my opinion, and these results could have happened by chance (p > 0.05) [9]. At the same time, it is worth mentioning that a breakfast could increase available blood glucose levels which could enhance memory due to more energy being available.

In the before-mentioned Ramadan study, the time of the day influenced how well people remembered words. They performed worse in the afternoon compared to the morning. But I’d like to mention again that Ramadan itself is a different type of fast. It involves a continuous caloric restriction over successive days. Furthermore, It’s also quite possible that it’s not a fast. Conceptually it’s just a restriction of hours when one is and isn’t allowed to eat. Therefore, some people could abide by it and yet continue to consume the same amount of calories.

To continue with memory – how about the geometric and the more spatial side of memory?

Chances are, you will be worse at remembering different geometric shapes if you are fasted, with this effect becoming progressively worse in the afternoon and evening. [7]

In general, there is a consensus that there aren’t any changes to our working memory while being fasted for a short duration. [1, 2, 6]


Processing speed

Let’s move on with processing speed. Here the measured thing is the speed with which you can process information that is being presented to you. If you are fasted, you will probably find it more difficult to process different stimuli with the same speed compared to when you would be well fed. However, this will actually come into play only when the tasks that need to be processed are of medium difficulty [12]. This offers an inverse U-shaped curve when it comes to fasting and processing speed with the difficulty of tasks that need to be processed. This means that there are no changes for processing easy or the most difficult tasks, there is, however, a decrease in processing speed for tasks of medium difficulty.

However, further research is warranted on this topic, as these are only two studies of smaller size.

Visual attention

Just one study was measuring visual attention. It observed that if you would be in a fasted state, you would probably react faster when presented with different visual stimuli. [6]


Executive functioning

Executive functioning is a general term which encompasses a multitude of cognitive processes that happen in our brain. Some of these are reasoning, mental flexibility, problem solving, planning, and execution of tasks themselves. Studies done to this point were testing increases or decreases in performance on tasks that involved interference. To put it short – they had to solve the Stroop task; if you’re not familiar with it – a series of words (BLUE, GREEN, RED etc.) are written in either the same color or a different color.

This presents an interference in the task which people have to “avoid”. Their job is to give the right answer – either the written color or the color itself. The number of errors people make when they have to write a certain color is then measured. In this case, it is possible that you would make more errors and need more time to decide about the right answer when fasted. With that said, there was only a general trend across the studies to support these findings. [7, 9, 10]

That means these results could have happened by chance (p > 0.05). Therefore, they are not necessarily conclusive.


Abstract reasoning

When talking about abstract reasoning, the main idea is to derive different concepts in an abstract way from their literal meanings. There was one study that tested this.

People were presented with a 2×2 matrix. One block from the 2×2 matrix was missing, their job was to complete it to create a coherent image. According to this study you would do worse here, having greater difficulty picking out the right remaining image. However, this one doesn’t agree – they noted no differences and came to the conclusion it doesn’t matter if you’re in a fasted state or not.

What does this tell us about abstract reasoning and fasting? Not much I’m afraid. This area is riddled with bad methodology and shitty science in general. It’s hard to generalize due to the different ways these studies were done.

The best word of advice is to keep an open mind and wait for more information to become revealed.


Take home messages

  • If you fasted for over 24h, your reaction times would probably increase. If it was one or two meals, as is common when doing intermittent fasting, there wouldn’t be any differences.
  • You wouldn’t experience any performance losses when talking about working memory or memorizing words. However, it is possible you would find it more difficult to remember different geometric shapes.
  • When talking about tasks of medium difficulty for you, you would probably need more time to process them when fasted.
  • It’s possible you will be more attentive to things when fasted.
  • When it comes to abstract reasoning and executive functioning, the results become more mixed and unsure. I would refrain from making any certain judgments.

Congratulations for finishing this post. As you can see, while intermittent fasting has multiple physical health benefits, there are some unwanted shortcomings that can affect our brains in different situations. It is obviously totally up to you and your personal goals in life as to decide whether these shortcoming will affect you in a significant way in your daily routine.

Will these effects stop you from doing intermittent fasting? Will they make you think twice before and when to do it the next time? Tell me what you think!

About Author

Sebastijan Veselic

BSc in Psychology, currently doing a MSc in Cognitive science. Pursuing and interested in many academic and scientific disciplines and topics, as well as some less so. These include, but are not limited to, cute cats on the internet.