If you spend all day trying to keep a lid on your garden's temperatures, it might seem a little crazy to switch on a heater at night. But keeping the difference between you daytime and nighttime temperatures small can really help to increase your garden's productivity.
The key to productivity when growing fruiting annuals indoors lies in the overall form of your plants. Tall, leggy plants are a nightmare to grow indoors, unless you want to rig up side lighting all over the place. Most growers prefer to hang their grow lights from the ceiling and beam light down to their plants from above. But if you have to raise your grow lights up high to accommodate tall plants, that means that most of your foliage lower down is receiving precious few photons (light energy.) Remember, grow lights are nothing like the Sun - you must always position your lights as close to your plants are possible without causing localized heat problems (typically leaf temperature should never go above 82 °F or your plants' leaves will start to shut down photosynthesis and go into self-protect mode.)
So, indoor gardens—the really successful ones that is—understand how important plant form is. A squat, wide plant is so much easier to illuminate effeciently using grow lights than a tall, leggy plant. In short, MORE of the plant overall basks in the "sweet zone" - where it's receiving maximum energy from your grow lights but not overheating (the classic "not too close, not too far" area!)
But what have nighttime temperatures got to do with all this? Well, it turns out that many plant species will develop into a squatter overall form if temperatures at night are close to those in the daytime. So, if your garden's temperature is 80 °F during the day, your plants will grow squatter and lower if the nighttime temperatures at in the 70s, not the 60s.
A fan speed controller with the ability to set a separate nighttime minimum is a great idea. But you should also look at investing in some block heaters, particularly if you live in a northern climate. Letting temperatures drop too low when your lights are out can stunt your plants and cause excess moisture to drop out of the air, risking the quick onset of mold problems.
Running your grow lights at night can help—not only does this help to keep your "lights on" temperatures lower, but you can often benefit from cheaper electricity tarrifs at night too. Also, ambient daytime temperatures can help make keeping your lights out temperatures easier to manage.
Some savvy growers create "flip rooms" - these are two (usually identical) grow rooms that are placed adjacent to each other. Both run 12 / 12 light cycles but alternately. So, when one room is ON, the other is OFF. Warm air from the room that is ON is exhausted into the room that is OFF, helping to equalise temperatures.