Depending on what thermostat you purchase they operate differently. You plug in your selected heat sources to the thermostat and select the desired temperature.Most commonly a thermostat has a temperature probe that measures the temperature at a point you have placed it within an enclosure. The thermostat will then take the actual temperature reading from the probe and relay it back to the main unit. The thermostat automatically adjusts the amount of power going to your heat sources to make the actual temp match your desired temp. All a thermostat does is adjust the amount of power that reaches your heating sources. It Automatically adjusts with temperature fluctuations to hold your desired temp. You can usually use a series of timers to control which power sources you wish to be operation during different times. This will solve the day night issue. Depending on your thermostat you can also program in additional features such as a night drop. A night drop chages the temp from a daytime level to a nightime level for breeding season and other purposes.
The best basic thermostat out there today would be the ranco assembled by boaphile. You can find it at:
http://www.boaphileplastics.com/thermostat.html
It is a little pricey but well worth the investment. I would highly suggest purchasing one. Email me if you have any questions at all I am happy to help my fellow herpers.
Jonathan T. Lane
Snake Lane Reptiles
chicagopythons@hotmail.com