On my laptop running Windows 8.1, there was an annoying bug that caused the monitor to stay at full brightness after sleep or hibernation. This caused loss of productivity since eyes get strained in short period if the monitor brightness is too high, especially in ambient lighted environments. The problem will normally be resolved by a system restart, but it is not practical to restart at all times.
I have tried several solutions including uninstalling and re-installing the display drivers.
Here is the solution that worked, albeit being temporary (till next restart). The step can be repeated until Microsoft fixes the bug in upcoming Windows updates.
1. Goto Device Manager
2. Expand the Monitor device. Right click on the monitor name and Uninstall the monitor.
3. Goto File > Scan for Hardware Changes
Monitor brightness will immediately reduce and the brightness settings will start working again.
There you go. Enjoy.