Night mode has been turned off by the developers because it was super easy to exploit. Since night mode activated based on your PC's region and time settings to "accurately" determine when night mode should active per account, players were able to just change the time on their PC's to midnight to get the 100% defense bonus and have an unfair advantage.
When they turn it back on, it should be based on the server time which, which they will start at 05:00 GMT to simulate 00:00 EST. You'll see it in the upper right hand corner of the screen where inbound attacks normally pop up. It will look like a moon surrounded by stars and be accompanied with a timer.