During the month of Muslim fasting (Ramadan) many people alter their sleeping habits and stay awake most of the night. We investigated the effect of this alteration on morning and midnight cortisol levels in 10 healthy adults in their homes. Four of the subjects showed alteration of the cortisol rhythm during the last 2 weeks of fasting with reversal of the morning/midnight ratio in some values. One lady was admitted for 24-h cortisol profile on Day 15 of Ramadan when the acrophase and nadir showed a forward shift by about 5-6 h, consistent with the shift of the subject's sleep. The morning cortisol returned to normal in all subjects 4 weeks after Ramadan. However, the midnight value was above 250 nmol/l in three of the subjects who exhibited the alteration during Ramadan. These findings suggest that single-point cortisol values can be misleading in many Muslin countries during or shortly after Ramadan.