What Causes Headaches Persistently For A Long Time?

I have been getting sudden headaches as I call them for about the past 14 years. ive been to the doctor for a cat scan and they found nothing wrong. i still get them but not as frequently, they come on very quickly and vanish about 30 seconds later. its more annoying than anything and I'm still clueless why this happens. could it be something to do with my work schedule (graveyard shift)?