Redshift

Redshift is a daemon that adjusts the color temperature of your screen depending on the time of day. Shifting your screen towards red at night is said to help your body clock wind down and prepare you for sleep.

Turning off redshift at night makes your screen seem blue/bright as fuck and will likely make you squint and recoil.

Some anons consider it snakeoil. It could be similar to the blue/black white/gold dress bullshit, where different people's eyes see things differently. Testing it out yourself is the only way to know.

The Arch Linux Wiki maintains a decent guide on installing and configuring it.

It is based on the multiplatform f.lux program.