the right Temperature…
Hedgehogs need their ambient temperature to be around 75-78°F. If the temperature drops too much below 75°F, your hedgehog might go into a false hibernation attempt, which most likely will result in its death. They can survive short periods in the 68-70°F range, but normal temperature should be kept around 75-78°F. You should monitor the temperature using a thermometer. You can purchase a digital thermometer at most stores in the kitchen department.
… and how to keep it there
You can do one of two things to keep your hedgehog warm. First is simply keep the entire room in the mid-70s. This is usually not a problem during summertime, but in the winter this can become expensive. The second option is to just heat the cage itself. Several methods that don’t work are: heating pads (human grade), reptile heat rocks, and anything using fire or chemicals. The best, and the recommended way to heat the cage, is with a ceramic heat emitter bulb. Infrared basking lamps work, but they put out an annoying (for you and the hedgehog) red light. Unfortunately, it is impossible to say what wattage ceramic heat emitter you should buy. Many factors go into determining the final temperature of the cage, such as proximity to walls or windows, airflow in the room, bedding used, accessories in the cage, and simply how warm the room is to start with. For example, if the room starts at 71°F and a 60 watt ceramic heat emitter can warm a cage up to say 75°F, that same ceramic heat emitter may only warm the cage to 69°F if the room started at 65°F, and a higher wattage ceramic heat emitter would be recommended. Here are some examples of ceramic heat emitters you can purchase. You will need the ceramic heat emitter bulb and the reflecting fixture. Just make sure the fixture is rated for the wattage of bulb you are purchasing. Remember, purchasing online is usually cheaper.