How do I regulate humidity?

My Hygrometer reads 34 low - 75 high. I grow 12/12 from seed and right now it's just about to stretch. In my past 5 grows, 2 of which I included a vegetative state, I've received some really low yields. I use a 400 watt hps 12 inches away from the canopy throughout the whole process when it's 12/12 and I throw in the MH when I do a vegetative state, either way I still get flimsy nugs. The Ph is kept at 5.8-6.0. and I use a hydroponic system. When going into flowering I give it the high phosphorus nutrient solution in the chart given by general hydroponics. I keep the temperatures ranging from 79-84 when the lights are on and down to 55-65 at night. If absolutely needed I can add a heater during the night; But as another user suggested, I believe it is more of a humidity issue.

Up until now i have not paid any attention to the hygrometer portion of my thermometer/hygrometer combo. I have just tried to make sure all of the leaves stay green and get no visible heat stress. Recently i've found out that plants need to be in different humidities as they go through different stages of life. On my current plant which is 12/12 from seed, the newest growth stopped growing in vertical height and is now forming lots of pistils so i'm assuming pre-flowering is done with and it's starting to bloom. What humidity should my grow room be in right now? and what are ways to being humidity down? I know I can just leave out open buckets of water to evaporate and increase humidity but i'm not sure how to bring it down.

A simple dehumidifier will keep humidity in check. You also need very good air movement and air exchanges. I assume no co2 usage. You can also use flower hardener additive to help dense up buds but good air flow and air exchange will help greatly

There is a fan running the whole 12 hours the lights are on and I leave the window to that room open the whole way. I grow in a closet and the window is on the opposite side of the room. I leave the closet doors open while the lights are on.

Leaving the windows open may be your issue. Your allowing outside moisture into the area so your loosing all control of the humidity you would have with it closed. Im not sure of the grow room size but in like a 4x8 tent 4 fans is minimal in a room 8x8 your gonna be looking at 4 to just move the air and then one to bring new air in and one to take the old air out. CFM should match the area size.

It's actually at night when the humidity goes out of the range it's supposed to and that's when the windows are closed. it's daytime here and I just checked, it's at 40% humidity. But at night it spikes up to 75%, so I think the dehumidifier is the solution.