Just remember to take breaks and you'll be fine, let your eyes rest and don't let annoying fuckers who don't know about desk work influence how you sit.
It really is just a case of making sure you take regular breaks and don't always work non-stop, in Jewellery we have to make sure we rest our eyes after awhile by staring at something far away because otherwise you're eyes end up going skewy and you go short sighted from staring at stuff that's less than 1mm in size for too long.
Sometimes it's really small stuff like this that can affect your productivity, I actually find I work much better taking regular breaks than I do trying to force myself to work for long hours, unfortunately corporate lifestyle doesn't really allow for this method of working so I'm trying to be self-employed
It's also been proven that being able to work independently with a much less rigid scale and even adding small things like plants in your workspace increases productivity just because of your mood.