In obese adults, a 24-hour fast shifts the peak time of the stress hormone cortisol earlier by about 42 to 57 minutes in those who naturally wake up early, but does not change the timing in those who naturally stay up late. This suggests that a person's internal body clock affects how their cortisol rhythm responds to fasting.

In obese adults, a 24-hour fast is linked to changes in the daily pattern of cortisol release, including a higher peak level and an earlier timing of that peak, while total cortisol production over the day remains unchanged.

In obese women, skipping food for 24 hours is linked to higher peaks and an earlier timing of the daily cortisol rhythm, but these changes do not occur in obese men, suggesting that fasting affects cortisol patterns differently by sex.

In obese adults on a calorie-restricted diet, a 24-hour fast is linked to a measurable increase in the daily peak of cortisol and a shift in when that peak occurs, without changing the total amount of cortisol produced over 24 hours.

During fasting, obese women experience a stronger and earlier peak in cortisol levels across the day compared to obese men, whose cortisol rhythm remains unchanged.

People who fast every morning over a long period show a less pronounced daily rhythm in cortisol levels and are more likely to have insulin resistance and metabolic issues.