Humidity rise at night

It happens because temps drop. The plant stop transpiring, but the temps drop. The "relative" in relative humidity applies here. It is very important you control this. Dewpoint is dangerous -- even if you get near it, you can cause condensation within the foliage where temps are a bit cooler and RH is higher than ambient in the tent/room it resides. If you consistently get condensation each night as lights go out, you will almost certainly grow some WPM (fungus) at some point... definitely high-risk. I know from experience. I thought i had it locked down with dehumidifier but it was not enough. A temp/rh probe allowed me to monitor the temps and RH even after lights went out in flower (photoperiods). I quickly learned my dehum was simply not strong enough to handle the rise in RH after lights out. You want a safe distance from dewpoint.. it's not just "stay above it.." youw ant a good 7-10F or more cushion, because the air around leaf is up to -5F offset and the rh is higher than the open areas of room. Measuring from within canopy is a good idea, if you can. A few spots paints the best picture. sounds like you could also temporarily increase your exhaust (intake?) fan a bit if the outside room is 40% that'll help if air is exchanged at a faster rate. I had to downsize my room or buy a 2nd dehum. I choose to downsize and do 2 grows/year. A 50pt dehum can keep up with 35-40sq ft of canopy in the context of awinter season, furnace on and a ambeint RH of 30-35%. To help size any needs. (that is the newer standard of rating dehums in USA, formerly a 70pt dehum before the rules changed)

As long as you vent out the moist air and bring in the 40% living room air it will be

You’re seeing this swing because you are talking about relative humidity which is directly effected by air temperature. The warmer the temperature the more humidity the air can hold. When the lights go off, the temp inside your tent drops thus the relative humidity will go up. Your best bet is to exhaust the humid air from the tent to the outside. Running your exhaust fan at a higher CFM speed will help remove that hot moist air. Keep steady turbulent air moving though your tent with fans and you should not have any issues. I hope this helps.

Relative humidity is the ratio of water vapor in the air to the amount of water the air could hold at the current temperature and pressure. Warm air can hold more water vapor than cold air. Relative humidity (RH) is expressed by the following equation: RH = water in air ÷ water air could hold (at constant temperature and pressure) The relative humidity is given as a percent. For example, if a pound of air at 75°F could hold 4 grams of water vapor, and there are only 3 grams of water in the air, then the relative humidity (RH) is: 3 ÷ 4 = 0.75 = 75% Water vapor moves from an area of high relative humidity to one of low relative humidity. The greater the difference in humidity, the faster water moves. This factor is important because the rate of water movement directly affects a plant's transpiration rate. The relative humidity in the air spaces between leaf cells approaches 100 percent. When a stoma opens, water vapor inside the leaf rushes out into the surrounding air and a bubble of high humidity forms around the stoma. By saturating this small area of air, the bubble reduces the difference in relative humidity between the air spaces within the leaf and the air adjacent to the leaf. As a result, transpiration slows down. If the wind blows the humidity bubble away, however, transpiration increases. Thus, transpiration usually is at its peak on hot, dry, windy days. On the other hand, transpiration generally is quite slow when temperatures are cool, humidity is high, and there is no wind. Hot, dry conditions generally occur during the summer, which partially explains why plants wilt quickly in the summer. If a constant supply of water is not available to be absorbed by the roots and moved to the leaves, turgor pressure is lost and leaves go limp. Your lights on should be during the night. Lights off during day when it hotter. A few options you vent outside. Negative pressure closed loop. (Difficult with your humidity levels if not impossible) Dehumidifier.

It's because it's RELATIVE humidity. Warmer air can carry more moisture than cooler air. So let's say the rh is 70% at 70°. When the lights go out and the temps go down the cooler air can carry less moisture so the % rh goes up. It means then that it will lose its ability to hold as much water and the dew point will be reached easier resulting in condensation I hope that helps. Just keep airflow moving and understand the difference between relative humidity and (actual) humidity