<Li> Chronic: occurring 15 times a month for at least 3 months (CTTH - chronic tension - type headache). </Li> <P> Various precipitating factors may cause tension - type headaches in susceptible individuals: </P> <Ul> <Li> Stress: usually occurs in the afternoon after long stressful work hours or after an exam </Li> <Li> Sleep deprivation </Li> <Li> Uncomfortable stressful position and / or bad posture </Li> <Li> Irregular meal time (hunger) </Li> <Li> Eyestrain </Li> </Ul> <Li> Stress: usually occurs in the afternoon after long stressful work hours or after an exam </Li>

What cause headache at the back of the head