Ask HN: Software that reminds you to get up to take break

I tend to hack non-stop for hours and ended up with stiff neck and lower-back aches, I have been told that I should be getting up every X hours to stretch but I find it's easy to just forget to do that when are you in the zone.

Is there a software you guys use that reminds you to get up every X hours or so with some popups or something? I am using Mac OS btw.