Hey friend,
Yes very normal. My plants always seem to transpire a fair bit at lights off, and even worse if i happen to water late in the day or towards the end of the day.
If you have fans keep em on at night, and if you have an exhaust try running it a few hours in the dark cycle to just flush that air out.
Something about the drop in temperature seems to send out moisture, i know that isn't overly scientific but it's the truth...it would happen both to my outdoor grow and indoors.
Happy growing ✌️