Night mode will go to 50% of whatever strength you are currently running in the mode you are running. So if it is set to reef crest it will stay in reef crest. If it is set to 50% it will reduce to 25%. You can change the percent reduction in configuration mode. You can also change the duration there too.
To disable night mode, unplug the Vortech and plug it back in. The night mode time will be forgotten.