Dejal Time Out! will gently remind you to take a break on a regular basis.

Time Out! has two kinds of breaks: a "Normal" break, typically for 10 minutes after 50 minutes of work, so you can move about and relax, plus a "Micro" break: a very brief pause of typically 10 seconds every 10 minutes, so you can remember not to tense up too much for long periods. You can disable either kind of break if desired, and the breaks are automatically paused when you go away from your computer, and can be reset when you come back.

You can configure how long each kind of break lasts, and how long between breaks. Each Time Out is announced via the screen slowly dimming, with related graphics materializing, and when the break is complete, it fades out again. You can change the time these transitions take... and you can even change the color and the level of transparency during the break. So if you like, you can make it mostly transparent so you can continue reading while on your break... though it's better for you if you give your eyes a rest during the Time Out.

Requirements:
OS X 10.10+
Intel

Also available is:
Intel version 1.7.1 for OS X 10.7-10.8