Trust me, you want Flux. Your eyes & sleep quality will thank you.

If you need to manually start the applet just type 'fluxgui' in terminal.

Resource: Flux Repo

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

Resource: Create a Cron Task in Ubuntu

Check if cron is installed:

dpkg -l cron

Install cron (if needed)

sudo apt-get install cron

Verify cron is running.

systemctl status cron

Create a new cron job.

crontab -e

Select your terminal editor of choice.

Resource: Crontab Generator

Use the Crontab Generator to select when you want fluxgui to run. FYI: 'Command To Execute' field on Crontab Generator is 'fluxgui'

Verify cron job was created.

crontab -l

If you need to manually start the applet just type 'fluxgui' in terminal.

Winning! You're good to go.