How to fix night light not working in Windows 10 May Update 2019

Lot of "fixes" have been suggested on Internet, but these two steps worked for me :

- Disable fast startup in Windows 10
- Reboot. That's it

Read the following if you don't know how to turn off Fast startup (enabled by default)

How to disable Fast Boot in Windows 10 ? 

1. Right-click the Start button. Click Search.
2. Type Control Panel and hit Enter on your keyboard.
3. Click Power Options.
4. Click Choose what the power buttons do.
5. Click Change settings that are currently unavailable.
6. Disable Turn on fast startup (recommended).
7. Click Save changes.

Enjoy Windows 10 Night Light again.