Aware is very simple app living in menu bar showing how long you have been using your computer. Its is good app which can help you to take breaks from long hours of working on computer. It knows this because it detects the movements of your mouse and the keystrokes on your keyboard. After a short time of inactivity (a break), Aware will pause the timer, then reset and start again when more activity is detected.
There are no assumptions made as to what you do with this information. No popups or alarms to tell you to take a break, just a record of time tracked in your menubar for easy access.
You can download it from here for App store.