That is not true. It is good to flush right as or right before the lights go on. Flushing right before a dark cycle had a higher chance for root rot as well as general problems with your plant. That is definitely not a N def. The leaves would be yellowing and then dying. Not just turning brown. Most likely the early stages of a burn. I would recommend a soil flush(make sure the last set of water is ph adjusted). Then let it dry out. After it has dried out, feed with 50% or less nutrients. If you follow this, you should be good. Nutrient burn is no joke. It is much harder to recover from than a deficiency. I speak from expierience.