Common headaches: type, duration, frequency and implications. Questionnaires completed by 327 preclinical medical and dental students showed that 97.9% had experienced headaches, most frequently attributed to insufficient sleep (38.8%), mental stress (38.8%), alcohol (38.5%), excess heat (36.7%), reading (31.5%), excess noise (29.9%) or light (27.7%), and sleeping too long (23.5%). The frequency and duration of these and other headaches are listed. 8% of the headache group had consulted a doctor. Only 2.1% of all students had never experienced a headache. Two hypotheses are examined: (1) can headaches be normal?--delineated by their disappearing soon after the noxious stimulus has ceased; (2) can pain in the head, as elsewhere in the body, act as a warning and therefore have protective, even survival value?. 