The most common causes of long term sickness is stress, one in five people suffers from workplace stress. HSE report (HSE management Standard and stress-related work outcomes) estimate between year 2007 and 2008 13.5 million working days were lost due to stress related absence.
Here is some of the possible cause of stress in the workplace
Bad working environment, too much responsibility, long hours, heavy workload
Poor communication, not being involve with decision making
Skill not matching which are needed for the job
More detail on cause of stress also you can find out if you are stresses by your job by doing short assessment at the NHS site.
In the following video a expert describe what is stress ,the common cause of stress and is symptoms.