When we slouch forwards we are effectively shrinking our chest cavity. In doing this we are reducing our lungs capacity and reducing oxygen intake. We need oxygen for our brain to concentrate and being stationary in this position is reducing brain function
Its not just our brains that are struggling either - some muscles are becoming shortened due to contracted for long periods of time such as the hip flexors and pectoral muscles but other muscles are being stretched and we are literally hanging off them. All the muscles that are being stretched along our backs are literally gripping for dear life!
Muscles are designed to move the joints in our bodies - all 360 of them. they contract to move the sides of a joint closer to each other and then relax to return the joint to its original position. This action encourages the blood to travel through and deliver nutrients and oxygen to the muscle itself and the nerves and structures around it whilst taking waste products away from the area. When we get numbness from sitting too long in a position it is due to lack of blood to the area which is why our bodies force us to move!
So what can we do as obviously we need to go to work! You could get a standing desk which would be ideal (although I do know some people who have these and still do not move!), more practically, you could have your desk ergonomically set up. Many large companies offer this service though their HR but if not, Mr Google will probably give you some good tips! It is all about setting up your desk so that you sit in an optimal position for your spine. By this I am talking more 'S' shaped than 'C' shaped.
It is recommended that you get up every 30 mins 2 mins of movement. When patients complain that they forget I always suggest they get up for a small glass of water after the first 30 mins and so long as they have another drink of water after their comfort break, their bladder will do all the reminding they need. Just making a conscious effort when you return to your desk to sit straighter can help too as even if it is only for 10 mins every hour, you have still improved your posture for one of the six poor quality sitting hours you have whilst working!