This was an issue, and very correct to consider, with sauna heaters that have a closed body, stones inside, and the top open exposing the top layer of stones there. This is true in wood-fired heaters and electric heaters. Additionally, if the sauna is poorly insulated, or has actual large gaps etc ventilation at the lower level, there is a lot of cooler air coming in. The hot air from the heater makes a gradient layer from the stone level up. However, this is greatly resolved by having the benches at the correct height, well insulated walls and roof, and controlled ventilation, and additionally if there is mechanical forced ventilation pushing air into the sauna from the ceiling level, like we have, evening out the temperature gradient, that helps. And important thing is the heater: Grid-type pillar heaters etc that are open from the sides will heat the sauna air at a much lower level, and really take away that need to raise the benches above the top of the heater. A great sauna can be made into many various spaces that may have various constraints, and for example a high inside height of the sauna in many cases is not possible to do, while would be ideal, and the sauna will work fine without having it.