Top Bar Timer is a simple program designed for Mac that lives in your Menu Bar and is very simple to handle. You can use this application whenever you need to set up a timer for your break from work and wish to know exactly when the countdown hits zero.
The utility doesn't provide you with an actual interface. Instead, it brings you a small menu (on your Menu Bar) where you can choose the preferred timer settings (1, 5 or 10 minutes) or shut down the application.
In my testing, the program worked flawlessly and played the sound notification as soon as the timer hit zero. What's more, the app used a very small amount of CPU resources while counting down the time.
Nonetheless, Top Bar Timer offers a limited number of features. For example, you cannot set the timer for a defined amount of time, pause and resume timer, etc.
Cutting a long story short, I'd say you should look for other countdown utilities for your Mac. Even though this application comes without a price tag, it's still too simple and offers a small amount of features.