RH raises when the temp drops. When the lights are on and it is warm they transpire more water, adding to the Absolute Humidity. Then the lights go out and it cools. The AH will remain the same, but the RH rises, since it is relative to the air temperature. Warm air has the capacity to hold more water, or AH.
Here's a fun fact, or rule of thumb that works out: For every 20 degree F the air temp goes up, the RH will reduced 50%. It works out pretty darn close. Check it out yourself. If in the morning, (outside weather-wise) it's 50 degrees and 80% humidity, when the temp hits 70 degrees the RH will be 40%.