Everyone starting fasting wonders whether they’re about to be unbearable for a few hours a day. The research is more reassuring than the folklore, with one caveat at the end.
What did the trials find?
In a randomized controlled trial of a hundred and twenty-two people adapting to a sixteen-hour schedule over ten days, there was no evidence of worse thinking or worse mood compared with a control group.1
People did report feeling less able to concentrate during the fasting morning hours than after they ate, which is a different thing from actually performing worse.
Over a longer stretch, a six-month trial comparing an eight-hour window, ordinary daily restriction, and a control group found no change in mood or quality of life in any group. Everyone’s mood started in a healthy range, so read it as reassurance rather than benefit.2
So why do the first weeks feel rough?
Fasting is a mild physical stressor and your body treats it as one. Pooling thirteen human studies, fasting was followed by a short-lived rise in cortisol, which the authors suggest settles over weeks.3
Then add hunger. A 2025 review found people doing time-restricted eating reported more hunger rather than less, and hunger is a perfectly good explanation for a short temper at four in the afternoon.6
Early irritability is usually hunger and novelty rather than fasting itself, and it usually fades.
When is it a signal?
Irritability that keeps getting worse over weeks is worth taking seriously rather than tolerating. The usual causes, in order of how often they turn out to be the answer:
- Not enough food in the window. By a wide margin the most common one.
- Not enough sleep, which fasting then amplifies.
- The window being longer than suits you right now.
- The week before your period, when this is expected and temporary.
What if you’re in perimenopause?
Mood changes at this stage have their own cause and their own treatments, and it’s easy to pin them on whatever you changed most recently.
In a study of nearly three thousand women, those who weren’t especially anxious beforehand were more likely to report high anxiety symptoms during perimenopause than before it.4
And among women with no history of mood problems, those who entered perimenopause were around twice as likely to develop significant depressive symptoms as those who stayed premenopausal.5
So if your mood has shifted meaningfully and it isn’t tracking with hunger or sleep, that’s a conversation with a doctor rather than an adjustment to your window.