All plants have an ideal temperature and humidity range that they grow best in. The temperature range may depend on the spectrum of light being used, as some spectrums are more efficiently used by leaves than others, requiring a warmer (and easier / less expensive to maintain) environment.
In any indoor growing area it is important to monitor and maintain appropriate temperature and humidity ranges for the plants you are growing. If you aren’t monitoring temperature and humidity in an enclosed growing environment, it is very easy to let temperatures rise to a point your plants will bake. When temperatures are kept too low, plants will not grow to their full potential. Humidity too low can stress plants and encourage pests such as spider mites, but too high and mold may be a problem.
Unless you are monitoring the temperature and humidity in your growing area, you are just guessing as to the conditions your plants are going through. A relatively inexpensive max/min thermometer/hygrometer can keep track of the minimum and maximum temperature and humidity for you so that you can better understand what conditions your plants are experiencing.
If you need to decrease temperature, venting hot air out of the top of a small growing area will generally work. If temperatures are too low, simply surrounding your growing area with cheap plastic sheeting layers can add the insulation you need to keep temperatures warmer. It is almost never necessary to add a heater to an indoor growing area with lighting, provided it is properly insulated.
When plants are sparse or you are ventilating frequently with dry air, increasing humidity in the growing area may be necessary. Many types of humidifiers are available, but in a small enclosed environment a shallow tray full of water, especially with a fan blowing across it, is often all you will need to increase humidity.
As your plants get larger and have more leaf surface area, humidity usually increases in the growing area. When humidity levels get too high for the plants you are growing, increase air exchange / ventilation to decrease humidity. If this is not enough to decrease humidity or results in temperatures falling too low, dehumidifiers are available that will remove excess moisture from the air.