The most common triggers that stimulate migraines are stress, weather changes, alcohol, changes in dietary or sleep patterns, and hormonal fluctuations, which cause "menstrual migraines" in some women. Other factors, such as caffeine, aged cheese, bright lights, exertion, smoke, and noise, are also triggers for some people. Triggers can be difficult to identify because it may be twenty-four to forty-eight hours between the trigger and the headache. Also, a particular factor may trigger a headache at some times but at other times may not. Many women have migraines when they are premenstrual but have them less often at other times.