If your sauna requires 120-volts to operate, it can use a standard 120-volt light bulb, rated at 60 watts maximum. For infrared saunas that require 230-volts, a 60-watt bulb is also required. Any bulb with a voltage rating from 210 to 250 volts will work, as long as it is rated at 60-watts or less. Simply unscrew the old bulb and replace with a new one.
Do not attempt to replace a 230-volt bulb with a standard 120-volt light bulb. It will burn the bulb out immediately and possibly damage the power supply.