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.

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.

During Ramadan fasting, cortisol levels at night tend to be higher than usual, while morning cortisol levels are lower, which may indicate a disruption in the body’s natural daily hormone cycle and could be related to changes in metabolism and sleep patterns.

During Ramadan fasting, when eating is limited to nighttime hours, levels of the sleep hormone melatonin decrease at night, and the daily pattern of the stress hormone cortisol shifts—rising in the evening and falling in the morning—which may affect sleep and metabolic processes.

Chronic skipping of breakfast disrupts circadian rhythms, leading to dysregulation of cortisol secretion and impaired insulin sensitivity.