When indoor weed plants are in the long dark cycles for flowering, a little extra dark does not affect the plants as much. So if for some reason you have to re-arrange the light timing schedule, just leave it DARK until the next new light cycle. During the flowering stage, indoor plants can easily handle no light for 48 hours and even longer. They do not grow as much and get pale or wimpy, but they will survive. Of it is possible, try to give them a few hours of any light during the cycle just to keep their internal rhythms in sync. If the marijuana plants are not in flowering but are in the vegetative cycle, you do not want the PFR dissipating in the longer dark cycle. To re-adjust during the vegetative cycle, leave the LIGHTS ON until the new dark time arrives. With marijuana plants in the vegetative stage, missing one cycle will not change their hormones much because it takes a week or so to change over completely. However if it looks like the neglect went on for a long time, get some light on them even a small gas lamp will do. If you have a crisis such as a power failure or light failure, remember the plants are are so sensitive that their leaves can sense light as well as the human eye. This means that in an emergency, any small amount of light, even a flashlight can give the plants the long day signal they need. However if you have lost your power I am guessing you are not reading this article.