Windows night light and brightness are not the same thing.
Windows night light changes temperature color of the screen, in turns it's color more warm or cold, this has to be changed directly in Windows or in the notification bar button as shown in your pic-1.
Brightness is the screen brightness, it decreases or increases with F4 or F5 independently of the mode being in night light or not.
Don't know if this was your doubt, anyway F4 and F5 keys don't change from night to day light mode or vice versa.
Check this Microsoft site for Night light automatic configuration
Set your display for night time in Windows 10On the other hand if you can't adjust screen brightness either with F4 or F5 keys and/or the slider, this is a known issue that can happen after a windows or driver update. A solution from Microsoft can be found here
Why can't I change my screen brightness with Windows 10?Or you can google it for 'windows 10 can't change brightness', and find other sites solutions for it.
Hope this helps.
