Do your knees fell like they are locked in a vice when you stand up? There is
only one answer to that, I'm afraid, and that's regular exercise away from the
keyboard.
Once you become your own boss, you may turn out to be a real slave driver. I
know many freelancers who work at home and who simply won't let up on
themselves—working hours that would be intolerable in the workplace.
The only time they get up from their desks is to grab a cup of coffee and a
donut. As your own boss and your own union rep, you have to negotiate something
better for yourself than that if you don't want to end up trying to meet a
deadline from a hospital bed.
Try changing the coffee and donuts to mineral water and apples. It's not easy
(trust me!) but start with just one coffee break and then work your way up. You
don't have to give up on coffee altogether, but less is better, and the best way
to avoid the donut and cookie syndrome is not to have them in the house. Just
look at them on the shelves and think, I slaved to get that check for this? Buy
yourself some aroma therapy instead.
The best tip of all is just not to sit there all day. Make it a practice to
get up every hour at least, walk around, stretch and exercise a bit, and have a
chat to someone, even if it's only your reflection in the mirror. Stagger your
workday so stuff that can be done away from the computer can be taken out into
the garden (not to the couch in front of the TV!) or the local park. Get a dog.
Walking an excitable young pup is the best exercise I know. If you have kids at
home, schedule some active playtime into your day. In less time than you think,
you will see positive benefits in the way you feel, and the way you work. A safe
workplace, and a refreshed body, mind and spirit, will do more to help you meet
those deadlines than all the coffee in the world.