A thermostat controls radiant heat under tiles.
According to the Department of Energy, radiant floor heating is "more efficient than baseboard heating and usually more efficient than forced-air heating because no energy is lost through ducts." Radiant heat usually involves the use of electric mats or a series of pipes that circulate heated water. My house wouldn't easily allow for a liquid system, but it's great for new construction. Ian's incredible builder, Dustin, installed a Suntouch mat just under my aluminum tile flooring.
Electricity isn't cheap, so I have the mat programmed (using the programmable thermostat seen here) to warm up just before I wake. The floor is charged during off-peak hours and then stays warm for most of the day. Because I am not heating my floor during the times when electricity rates are at their peak, I save on energy costs while enjoying an indulgently warm bathroom.