Our body didn't evolve for all these centuries to end up seated on a chair for a whole day.
If you think about it, not so long ago, we had to push our body beyond limits to have something to eat.
Our visual capabilities were necessary to reach things that were more than 50cm way.
So all this energy, all these chemical reactions are still happening but not used and our body doesn't actually know what to do with them.
On top of this we're not helping it.
All this junk food (even the one you think is healthy most of the time is junk on its essence) even poisons us more and it's an overhead on top of everything else the body needs to be able to dispose every day.
Keeping yourself physically active is very important.
Now, as far as the brain is concerned, some studies say that if we don't use certain connections in our brain they will eventually be disconnected.
So the idea is to keep pushing your brain to do new things so you can build new connections, new routes between the neurons keeping you more intellectually active.
Speaking about myself, I find it very easy to be glued to a computer for several hours in a row.
If I get tired of doing something I can always do something completely different, that will stimulate my brain again, but without actually leaving the computer.
This is actually really bad. I realize this by simply going for a run.
Stressing my body physically really dumps all the accumulated stress and makes me feel really good.
Some people feel the same thing while playing some instrument, reading, doing a puzzle... it really depends on your body but I should find what best suites you.
In my spare time, or not even spare i will always have my ear plugged in with music, that help me in building my concentration, helps from distraction; don't have to worry what is my neighbor talking or going on. When total free i love to watch some good movies