I'm assuming you have African Pygmy Hedgehogs and not European ones, correct?
18-20 degrees Celsius (65-69ish for the Fahrenheit challenged, like myself who had to use a converter
) is a little too cold for hedgehogs. Those temperatures can actually lower the immune system and the hedgies can get sick easier--if they don't hibernate first.
23-27 degrees Celsius (73-80) is a more suitable temperature.
You should try to keep the temperature constant all the time, day and night. You can buy ceramic heat emitters to do so, or you can keep the whole room that temperature using a space heater. Some people use heating pads, but those should only be placed under half the cage and I really feel that it's better to have the whole environment warm, not just parts of it.
I typically have my hedgie's environment around 24 (75) day and night.