Sauna lights are a small line on a build and the one people most often get wrong, because an ordinary fitting in a cabin at full temperature is both a failure and a hazard. What you need is a luminaire rated for the heat and the humidity, sited where it is not in your eyes and not in the hottest air.
The fitting has to be rated
A sauna luminaire is specified for high ambient temperature and for moisture, with a heat resistant gasket and glass rather than plastic. Ordinary fittings fail in months: the diffuser yellows and warps, the gasket hardens and the lampholder degrades. This is not an aesthetic preference, it is the difference between a fitting that lasts and one that becomes a fault.
Where to put it
Not overhead in the hottest air and not facing you. The conventional places are low on a wall behind or under the upper bench, or in a corner behind a timber shade, so the room is lit and the source is not in the line of sight. Indirect light from below reads as warm and comfortable; a bright ceiling fitting does not.
LED, fibre optic and the alternatives
LED strip rated for the temperature is now common and works well under bench edges, provided the driver lives outside the cabin where it is cool. Fibre optic keeps every electrical part outside entirely and is the neatest solution in a wet cabin, at a price. Salt lamps and color therapy panels are preference and carry no established benefit.
The wiring
This is a hot, sometimes wet room, so the wiring is specified for it: heat resistant cable, the switch outside the cabin, and the whole circuit installed to code by a licensed electrician along with the heater. Do not run ordinary flex through a hot ceiling void and do not put a switch inside the room.
Questions people ask about sauna lights
Can I use a normal light fitting?
No. It needs to be rated for high ambient temperature and moisture. An ordinary fitting will fail and is a hazard while it does.
Where should the light go?
Low on a wall or under the upper bench, out of the hottest air and out of your line of sight. Indirect light suits the room.
Should the switch be inside?
No. The switch goes outside the cabin, and the wiring is heat resistant cable installed to code with the heater circuit.