Fasting and Mental Health: Why Many People Experience a Mood High and When Fasting Is Not Good for Your Mind

Maybe you know this. The first two days without food are tough. Your stomach growls, your head throbs, and you get irritated more quickly than usual.

And then, at some point, something shifts.

Maybe you wondered afterward what had happened. Or whether fasting could also do something about your darker phases. If you recognize yourself here, you are not alone.

I want to show you two things. First, what might be behind this high and which parts of it are supported by evidence. Second, when fasting is not good for your mental health, because exactly this good feeling can become a trap.

You can read how a fast lasting several days works in Buchinger therapeutic fasting. What ketosis is and how to measure it is covered in Ketosis during fasting: how to recognize and measure it. This article is about your head and your mood.

The fasting high: what many people experience between day two and day seven

In short: During a fast lasting several days, many people experience an elevated, clear mood, usually between day two and day seven. Reviews and large observational studies from fasting clinics describe this. Controlled studies are rare and small. Where the high comes from has not been clarified in humans.

The fasting high is not an internet myth. Clinics that work with fasting describe it often. What is surprising is how little we still know about it.

What does this look like in large numbers? There is an observational study on this from a fasting clinic on Lake Constance.

Is the high perhaps just joy about the number on the scale? A study from Essen argues against this.

A small pilot study by the same research group fits the picture. Thirteen healthy people without excess weight fasted at home for one week. Afterward, the sleep lab recorded fewer arousals during the night. The participants reported better sleep, better concentration, more vigor and more emotional balance. Here, too, there was no comparison group.

Important for context: the fasting high is described mainly with fasts lasting several days, such as therapeutic fasting. For intermittent fasting with shorter breaks from eating, the data on mood look different. More on that below.

From anthroposophic medicine, I know a mental model that I find useful here. It distinguishes three faculties in a person: thinking, feeling and willing. The fasting high touches all three: thinking becomes clearer, feeling lighter, and the will experiences itself as strong. This is not a study result, but a lens. It becomes important when we come to the shadow side.

So that you always know where we stand, I separate three levels.

And now you know why your clear head on day three is real, but not yet an explanation.

Why the first days make you irritable and then hunger quiets down

Before the high comes, there is often a low. On day one and day two, many people are irritable, unfocused and thin-skinned. The people you live with sometimes notice it before you do.

A London study shows how mixed even short breaks from eating can be. Fifty-two women with an average age of 25 fasted for 18 hours and kept a diary. Irritability rose. At the same time, their sense of achievement, pride and control grew. Overall mood did not change significantly. We will need this finding again later.

In a small study from Bournemouth with 16 women doing intermittent fasting, the lower mood on fasting days was not linked to hunger. It was linked to distraction, meaning how hard it was to stay focused. On those days, the women also rated their own work performance as worse.

What happens in your body during these days? Imagine your liver as a pantry. In the first hours, you live on its supplies, the glycogen. Once the pantry is largely empty, your body switches over.

A team from the University of Florida and the National Institute on Aging, led by Anton and Mattson, calls this moment the metabolic switch. It typically occurs beyond twelve hours without food. Fatty acids are mobilized, and the liver uses them to build ketone bodies.

The most important ketone body is called beta-hydroxybutyrate. It can serve your brain as fuel. The Harvard physician Cahill described how exactly this ability allowed humans to survive long periods of starvation.

In the Überlingen cohort, mild complaints such as headaches, sleep disturbances and hunger occurred mainly in the first days of fasting. You can read more about sleep and fasting in Intermittent fasting and sleep. How to recognize and measure ketosis is covered in Ketosis during fasting: how to recognize and measure it.

And now you know why the low at the beginning can often be followed by a high.

Ketones, BDNF, serotonin: what might happen in the brain

Maybe you have read that fasting is a natural antidepressant. The idea sounds tempting. Let's look at what it is based on.

Beta-hydroxybutyrate is more than fuel. In a review, Newman and Verdin from the Buck Institute also describe it as a signaling molecule. It can be linked to the regulation of genes, meaning which blueprints a cell is currently reading.

Imagine ketone bodies as couriers that do not just deliver packages. They also bring notes with instructions.

One of these notes might concern BDNF. This is a growth factor for nerve cells, something like fertilizer for connections in the brain. BDNF is associated with better learning and with relief of depression and anxiety.

The neuroscientist Mattson from the National Institute on Aging describes a pattern he calls metabolic switching. A challenge involving ketosis, such as fasting or endurance exercise, is followed by recovery with food, rest and sleep. This alternation could make the networks for thinking and mood more resilient. Much of this comes from animal experiments.

A British review by Seidler and Barrow analyzed 82 papers on intermittent fasting and BDNF. In animal models, BDNF rose consistently with intermittent fasting, and cognitive performance improved. For humans, the authors consider further research necessary.

And serotonin, the famous messenger of mood? This is where it gets contradictory.

The review by Fond summarizes which mechanisms are being discussed: changes in messenger substances, sleep quality and the production of growth factors for nerve cells. None of them has been shown to be the cause of the high in people who fast.

Through the KPNI lens: four systems in conversation

Another bridge leads through the immune system. A team from the University of Cambridge led by Osimo and Khandaker analyzed 37 studies with 13,541 people with depression. In 27 percent, the inflammatory protein CRP was above 3 mg/L, and in 58 percent it was above 1 mg/L. Inflammation could therefore play a role for a large proportion of people with depression.

And fasting? At Mount Sinai in New York, a team led by Jordan and Merad showed, mainly in mice, that short-term fasting can markedly reduce inflammatory immune cells in the blood called monocytes. A meta-analysis of 18 randomized trials found only a slight reduction in CRP with intermittent fasting and calorie restriction, by 0.024 mg/dL on average. The messengers TNF-alpha and interleukin-6 did not change significantly.

Through the lens of Clinical Psychoneuroimmunology, four systems interlock during fasting. None of them explains the fasting high on its own.

And now you know why the explanation for the fasting high is more exciting than any headline, but still open.

Can fasting ease anxiety and depressive symptoms? What studies show

If you are going through a difficult phase, any prospect of relief sounds tempting. That is exactly when you deserve an honest answer instead of a promise.

In short: In two meta-analyses, depressive symptoms decreased slightly to moderately with fasting, and anxiety decreased in only one of the two. Almost all of the people studied had no psychiatric diagnosis. Good studies in people with depression or an anxiety disorder are lacking. That is why fasting is not a treatment for mental illness.

A 2023 systematic review by a team from São Paulo led by Murta comes to a clear finding. The human studies mainly examined healthy volunteers, and they were small, very heterogeneous and often had no control group. Whether intermittent fasting can contribute anything in depression that requires treatment still has to be clarified by better designed studies.

Such studies are only just beginning. In 2026, the protocol of a randomized trial from Tabriz was published, planned with 90 people with depression and obesity. There are no results yet.

The 2013 expert guideline on fasting lists mild to moderate depressive moods among the possible areas of use for medically supervised fasting. This reflects experience from fasting clinics. It is not proof of effectiveness from controlled studies in people with depression.

And now you know why the studies on fasting and mental health sound encouraging but do not yet give an answer for people with depression.

When fasting is not good for your mind: eating disorders, underweight and acute crises

Now comes the other side. The good feeling during fasting can become a trap for some people. Precisely because it feels so good.

Do you remember the London study? After 18 hours without food, the women felt prouder, more accomplished and more in control. The authors see this as a parallel to explanatory models of anorexia. In those models, starving oneself is also maintained by exactly these kinds of feelings. At the same time, they caution against equating short-term fasting with anorexia.

This is where the model of thinking, feeling and willing comes back. When the will takes the lead and control itself becomes the goal, fasting can lose its meaning. Then it no longer nourishes clarity, but inner pressure.

A Canadian survey of 2,762 adolescents and young adults shows how widespread the topic is. In the past year, 47.7 percent of the young women, 38.4 percent of the young men and 52.0 percent of the transgender and gender-diverse participants had practiced intermittent fasting. In all groups, intermittent fasting went along with more eating disorder symptoms, and for eating disorder behaviors the association was most consistent in women. A survey like this cannot clarify what is cause and what is consequence.

There are also reassuring data. A review from the University of Sydney found no consistent link between medically supervised, severe calorie restriction and new binge eating in people with excess weight and no history of binge eating. So what seems to matter is who fasts, why and in what setting.

And then there is a difference between fasting and starving. In the Minnesota experiment led by the nutrition researcher Keys, 36 conscientious objectors lived on severely reduced food for an extended period during the Second World War. Most lost more than a quarter of their body weight. Many became exhausted, listless and irritable. A limited, supervised fast is something different from long-term deprivation. If you are underweight, however, this line can be crossed quickly.

In an acute emotional crisis, your nervous system needs stability: regular meals, sleep and people around you. Fasting, by contrast, is an additional stimulus for the stress system. In the Essen study, cortisol rose by 17 percent during fasting, and in the Überlingen cohort, sleep disturbances occurred mainly in the first days. This is not a moment for self-experiments.

If you hesitate at any of these questions, that is not a failure. It is important information. In that case, talk to your doctor or your therapist first.

And now you know why the same high can be a gift for one person and a risk for another.

What you can do now: three levers for your head and your mood

You will not get fasting rules here, but three directions. They start where studies and experience point most clearly.

And now you know why a mindful approach to fasting says more about your mental health than the number of days you fast.

Frequently asked questions about fasting and mental health

You can read about the physical risks that longer fasting can carry in Water fasting: potential benefits and risks. How to end a longer fast gently is covered in Breaking a fast properly.

Where this topic touches other areas

Fasting and mental health touch on the nervous system, stress hormones, inflammation and lifestyle. If you want to go deeper, these neighboring topics from other guides are worth a look.