The best alternative to f.lux is Redshift, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 50 alternatives to f.lux and seven of them are so hopefully you can find a suitable replacement. Other interesting alternatives to f.lux are SunsetScreen, Twilight, Windows Night Light and Lumen.
f.lux alternatives are mainly Color Temperature Tools, but if you're looking for
Screen Dimmers you can filter on that. Other popular filters include
Linux,
Android,
Windows,
Screen Dimmer and
Mac. You can also filter by region, for example
EU-based alternatives if you prefer software developed in the European Union. These are just examples - use the filter bar below to find more specific alternatives to f.lux.
This is open source, while f.lux wants you to download a binary and run it constantly on your system.