Wisconsin Statute 347.489(1) states that if you ride your bike in the dark, you must have a white front light (either attached to the bike or to yourself, e.g. as a helmet light) as well as a red reflector or red light in the rear. This is the full text of the statute:
347.489 Lamps and other equipment on bicycles and other vehicles and devices.
(1) No person may operate a bicycle, motor bicycle, personal delivery device, or electric personal assistive mobility device upon a highway, sidewalk, bicycle lane, or bicycle way during hours of darkness unless the bicycle, motor bicycle, personal delivery device, or electric personal assistive mobility device is equipped with or, with respect to a bicycle or motor bicycle, the operator is wearing, a lamp emitting a white light visible from a distance of at least 500 feet to the front of the bicycle, motor bicycle, personal delivery device, or electric personal assistive mobility device. A bicycle, motor bicycle, personal delivery device, or electric personal assistive mobility device shall also be equipped with a red reflector that has a diameter of at least 2 inches of surface area or, with respect to an electric personal assistive mobility device, that is a strip of reflective tape that has at least 2 square inches of surface area, on the rear so mounted and maintained as to be visible from all distances from 50 to 500 feet to the rear when directly in front of lawful upper beams of headlamps on a motor vehicle. A lamp emitting a steady or flashing red light visible from a distance of 500 feet to the rear may be used in lieu of the red reflector.
Wisconsin Statute 347.489(1)
This is the legal minimum requirement, but it’s probably a good idea to have an actual light and not just a reflector in the rear. Also keep in mind that certain lights can be very irritating to other people on bikes or on foot, especially when they are aimed to high or flashing rapidly.