Many scientists believe that migraine headaches occur when arteries to the brain constrict, then dilate, resulting in the throbbing pain of a migraine headache. It isn't known what causes the blood vessels to constrict and dilate, but abnormally low blood levels of a chemical substance called serotonin may trigger the contractions. Migraine attacks can be disabling. More than 80% of migraine sufferers report at least some headache-related disability: 50% or more report mild or moderate disability, such as being unable to work or engage in usual daily activities; and about 30% report severe disability, sometimes requiring bed rest.