For your eyes to remain fully operational as long as possible you need to look away from the screen or paper at intervals. This is to allow the ciliary muscle to relax.
You can relax your ciliary muscles yourself without this tool, just follow the 20-20-20-20 guideline: Every 20 minutes of screen time look at an object at least 20 feet away for 20 seconds. The last 20 is a recommendation to blink 20 times to prevent eyes from getting dry.
This tool is just a whistle for you that tend to get encapsulated in your work or gaming. A whistle blows every 20 minutes to remind you that it is time to do the 20-20-20-20 exercise. It also blows a whistle when 20 seconds of relax has alapsed so that you don't need to change context.
The interval and pause times can be easily changed, should you feel your eyes need different settings.
To try this app - click a button below.
Images from the app