Classically the headache is unilateral, throbbing, and moderate to severe in intensity. It usually comes on gradually and is aggravated by physical activity. However, the effects of physical activity on migraine are complex and some researchers have concluded that, while exercise can trigger migraine attacks, regular exercise may have prophylactic effect and decrease frequency of attacks. The feeling of pulsating pain is not in phase with the pulse. In more than 40% of cases, however, the pain may be bilateral and neck pain is commonly associated with it. Bilateral pain is particularly common in those who have migraine without aura. Less commonly pain may occur primarily in the back or top of the head. The pain usually lasts 4 to 72 hours in adults, however in young children frequently lasts less than 1 hour. The frequency of attacks is variable, from a few in a lifetime to several a week, with the average being about one a month.