I've been using an app called "Twilight" to help with my circadian rhythm. I cannot recommend it enough (no root required): https://play.google.com/store/apps/details?id=com.urbandroid...

All the ones I've seen are different to f.lux. They seem to be additive - black becomes lighter, rather than just decreasing the blue percentage. I'd assumed it's a limitation of the filter API.