Working in front of computer may sounds as the most risk-less job in the world, unfortunately it's not. Your back, neck and wrist is the potential victim of RSI (repetitive strain injury or in other words : injury from staying on a same (stressfull) position for a long period. You most probably are already aware of this, but do you know that there are applications in Ubuntu repository to avoid this problem?
There are two applications to avoid RSI, they are :

• workrave : an utility for both KDE and Gnome that assists in the recovery and prevention of Repetitive Strain Injury (RSI). The program frequently alerts you to take micro-pauses, rest breaks and restricts you to your daily limit. And not just limited to one machine, it has network capability to monitor your activity across different computers.
  
• rsibreak : an utility for KDE4 to remind you to take a break, a short and long one. Simpler than workrave.
  

Just open your Synaptic Package Manager or KpackageKit, to install.
And while you're at it, why not install Gnaural, a programmable audio generator that works as an aural aid to meditation and relaxation, works best with headphones.
So while you are having a break, listen to Gnaural, and then get back to work fresher and calmer. For free.
And I keep this roller-massager handy, it's a back-massager but works well for shoulders and arms too, unfortunately it's not in repository :)

wooden roller-massager

keyword : RSI, stress, ubuntu