May I suggest RedShift?

It's more maintained than flu.x, it has more options and it works perfectly with Ubuntu 14.10 x64 and Ubuntu 15.04 x64.

It can be installed from the repositories (12.04, 14.04, and newer):

sudo apt-get install redshift redshift-gtk

You may optionally create a configuration file for RedShift. It is NOT created automatically, so you'll have to create it using gedit ~/.config/redshift.conf.

This is how my redshift.conf file looks like:

; Global settings for redshift
[redshift]
; Set the day and night screen temperatures
temp-day=4500
temp-night=3500

; Enable/Disable a smooth transition between day and night
; 0 will cause a direct change from day to night screen temperature.
; 1 will gradually increase or decrease the screen temperature
transition=1

; Set the screen brightness. Default is 1.0
;brightness=0.8
; It is also possible to use different settings for day and night since version 1.8.
brightness-day=0.9
brightness-night=0.7
; Set the screen gamma (for all colors, or each color channel individually)
gamma=0.8
;gamma=0.8:0.7:0.8

; Set the location-provider: 'geoclue', 'gnome-clock', 'manual'
; type 'redshift -l list' to see possible values
; The location provider settings are in a different section.
location-provider=geoclue

; Set the adjustment-method: 'randr', 'vidmode'
; type 'redshift -m list' to see all possible values
; 'randr' is the preferred method, 'vidmode' is an older API
; but works in some cases when 'randr' does not.
; The adjustment method settings are in a different section.
adjustment-method=randr

; Configuration of the location-provider:
; type 'redshift -l PROVIDER:help' to see the settings
; ex: 'redshift -l manual:help'
[manual]
; set these values if you've set the location-provider to manual instead of geoclue
;lat=51.522698
;lon=-0.085358

; Configuration of the adjustment-method
; type 'redshift -m METHOD:help' to see the settings
; ex: 'redshift -m randr:help'

[randr]
screen=0

If you need to, compiling it manually is also quite easy. Here is the official repository: https://github.com/jonls/redshift

Just make sure that you've installed all the dependencies specified in the travis.yml file before running the bootstrap executable file.

Instructions here: https://github.com/jonls/redshift/blob/master/HACKING.md

For Ubuntu 15.04 users: it could be that you won't be able to use redshift because of some missing dependencies. Try to compile it by getting the code directly from github.

sudo apt-get install build-essential libxcb-randr0-dev
./bootstrap
./configure --enable-randr
make
sudo checkinstall

...or just install if you don't want to use checkinstall. The libxcb-randr0-dev package should satisfy the dependency to use randr as an adjustment method. Otherwise try to enable vidmode by doing:

./configure --enable-vidmode

Here is another way I just found. I had to do this way because company firewall won't let me add apt repository no matter what I tried.

1. download source code from author's github

   git clone https://github.com/xflux-gui/xflux-gui.git
   

2. install

   cd xflux-gui
   sudo python setup.py install
   

3. run from command line

   fluxgui
   

[update as of Feb 23 2017] repo is changed

##Ubuntu 17.10 and later

Starting with GNOME desktop environment 3.24, which was released on March 22, 2017, a new Night Light feature is included that automatically reduces the amount of blue light emitted by screens during certain times of the day. The new feature can be enabled from the display settings. The screen color follows the sunrise/sunset times for your location, but it can also be set to a custom schedule. The Night Light panel indicator shows when the feature is active, and the system menu allows it to be temporarily disabled.

In Ubuntu 17.10, Ubuntu ships with GNOME desktop environment, not Unity, and the Night Light feature is included by default. Night Light works with both X11 and Wayland. To enable Night Light in Ubuntu 17.10 go to System Settings -> Devices -> Displays -> Night Light and slide the Night Light slider from OFF to ON. Then configure the schedule settings.

^{Night Light settings in Ubuntu 17.10}

The Night Light Slider GNOME Shell Extension provides an easy interface to tweak the temperature of the night light from the notification area of the panel. Be sure to check out the preferences in GNOME Tweak Tool to customize or enable added functionality. You can also easily configure the night light to always be on or to always show the status icon.

###Ubuntu 12.04-17.04

sudo add-apt-repository ppa:nathan-renniewaldock/flux
sudo apt-get update
sudo apt-get install fluxgui 

###Ubuntu 12.04-14.04

Installation is of f.lux in Ubuntu 12.04 and 14.04 is easily done by adding the PPA for f.lux. Simply type the following in your terminal:

sudo add-apt-repository ppa:kilian/f.lux
sudo apt-get update
sudo apt-get install fluxgui  

###Installing xflux daemon terminal program from the official f.lux website

64-bit

wget -c https://justgetflux.com/linux/xflux64.tgz
tar -xvzf xflux64.tgz
rm -rf xflux64.tgz
sudo cp xflux /usr/bin/
sudo chmod 755 /usr/bin/xflux

32-bit

wget -c https://justgetflux.com/linux/xflux-pre.tgz
tar -xvzf xflux-pre.tgz
rm -rf xflux-pre.tgz
sudo cp xflux /usr/bin/
sudo chmod 755 /usr/bin/xflux

f.lux GUI can be started from the Dash or from the terminal with the command fluxgui. When it is running there is a f.lux icon in the notification area of the panel.

⠀^{f.lux indicator applet preferences in Xubuntu 14.04}

New Flux app for Ubuntu 15.04+

Kilian Valkhof's repository for Flux does not update anymore, but you can use Nathan Rennie-Waldock’s PPA.

Run these commands to install Flux:

$ sudo add-apt-repository ppa:nathan-renniewaldock/flux
$ sudo apt-get update
$ sudo apt-get install fluxgui

To remove:

$ sudo apt-get remove fluxgui

Via: Ubuntu Handbook

May I suggest eyesome?

eyesome

Eyesome is a bash script running as a deamon and sleeping most of the time 24/7. It automatically adjusts screen brightness (and optionally gamma too) for your laptop display via hardware interface and up to two other monitors using xrandr's software control.

At sunrise (the time is automatically obtained from the internet each day), your screen brightness (and optionally gamma too) is adjusted gradually. The gradual adjustment is defined by you but, 120 minutes works for me. To keep the adjustments unnoticeable set a sleep interval between adjustments. Anywhere between 15 and 60 seconds is probably best and the default is 60.

After sunrise transition is complete, eyesome daemon sleeps many hours until sunset transition starts. I'm using 90 minutes before sunset but you can set any period you like.

Inversely to sunrise transition, the sunset transition gradually decreases screen brightness (and optionally gamma too) so it is unnoticeable.

Note that during nighttime transition gamma may increase. For example Red gamma may be defined as 1.0 during day and 1.2 during night to reduce eye strain. Blue gamma in turn may be defined as 1.0 during day and .8 during night so it will decrease instead.

To reduce resources, eyesome sleeps the entire period between sunset and sunrise. Depending on where you live and the season of the year, the average sleep will be 12 hours.

Eyesome Setup - Main Menu

To configure eyesome, a main menu is provided:

Edit Configuration - General tab

When you click the Edit button from the main menu the edit configuration general tab initially appears as shown below.

Your country/city name should automatically appear. If necessary you can override it.

Edit Configuration - Monitor 1 tab

Clicking on Monitor 1 tab above will reveal this panel in my configuration (yours may be different):

Don't be daunted by these settings they are for the most part automatically obtained by eyesome. You will need to set the daytime and night brightness/levels though.

Edit Configuration - Monitor 3 Tab

Clicking on Monitor 3 Tab reveals this panel in my configuration (yours may be different):

Monitor 2 Tab is not shown because it is a new TV with adaptive brightness and Smart OS. It requires no overrides by Eyesome.

If after eyesome is installed you attach a different monitor to your system you may have to enter the xrandr monitor name.

Eyesome Setup - 5 second test

From the main menu you can test your daytime and nighttime brightness and gamma settings for 5 seconds by clicking the Daytime and Nighttime buttons respectively. You can change the duration of the test from 5 seconds up to 20 seconds from the Edit Configuration - General Tab.

Here's what the 5 second Nighttime test looks like:

Unusual event handling

Assume you suspend your laptop when it's morning before work and the screen is at full dim. You come home after work when the sun is high in the sky and open your laptop. The screen is so dim you can't read it.

To address this scenario a systemd control file is provided:

• /etc/systemd/system-sleep/systemd-wake-eyesome control file is called whenever the system suspends or resumes.
• The control file calls the bash script /usr/local/bin/wake-eyesome.sh to reset brightness to full and then sleep until sunset transition.

You are watching a movie on your external TV at night and close your laptop lid for better viewing. Ubuntu / Lightdm / xrandr takes a few seconds and then resets your external TV to full full brightness. OUCH to your eyes.

To address this scenario an acpi event control file is provided:

• /etc/acpi/event/lid-event-eyesome control file is called whenever the laptop lid is opened or closed.
• The control file calls the bash script /etc/acpi/acpi-lid-eyesome.sh to handle the lid opening and closing.

• In turn the eyesome bash script calls /usr/local/bin/wake-eyesome.sh to reset brightness for nighttive viewing and then sleeps until sunset transition.

  Suspend/Lid close/Test brightness/power off/hotplug

Summary

Eyesome can be downloaded from: https://github.com/WinEunuuchs2Unix/eyesome

This program was just released in September 2018 so please let me know if you find any problems or have suggestions for improvement.

The documentation phase is just starting so don't hesitate to ask any questions. Your questions may even result in documentation improvements.

In ubuntu 20.04, you don't need to install any external packages. It has inbuilt package named xgamma. You can use it as follows:

xgamma 0.5

or you can adjust only blue color:

xgamma -bgamma 0.5

Its value can vary from 0 to 1.