To be nude in the sauna is for hygienic reasons a matter of course for most people in Germany and in countries, where sauna was invented (Scandinavia, Russia, etc.). However, in Scandinavia and Russia, men and women go to saunas separated by gender more often than in Germany – to be in a sauna all together in the altogether is the norm in predominantly German-speaking countries.1)
The majority of saunas in the countries mentioned above have usually a ban on clothing, and those who go to the sauna with their bathing suits are usually instructed by the staff to take them off and leave them outside – or to leave.
This is not the case in other countries. In many European countries, such as England or Italy, it is customary to go into the sauna in swimwear, there is even a clothing requirement in many saunas! However, nude vacationers from Germany are usually tolerated. Occasionally, however, there are already in Bavaria saunas with clothing obligations, which is also enforced there.
A newspaper ad intended to advertise the installation of a home sauna showed two people sitting nude on a sauna bench in the dim light from the side. This ad could only be published in England, after the two persons in the photo got photo-added bathing suits!
For medical reasons, a sauna visit is actually preferable to be done in the nude. The most synthetic fabrics of swimwear may cause skin irritation in the sauna and unhealthy increase in skin temperature – both may limit the well-being in the sauna, although not usually to a serious extent.
However, the often mentioned hygienic reasons for a nude sauna visit do not exist: the sauna visit is far too short, that the multiplication of bacteria and microbes on the skin could be relevant. It is important, however, that before the sauna visit (and also before swimming / bathing), you strip nude for the shower, to even perform a thorough pre-cleaning.
1)) Jeff Jarvis, an US American Internet guru and professor of media studies in New York, commented on this peculiarity of the Germans in an interview with the German newspaper Tagesspiegel: “The Germans have a much more mature relationship to their bodies.“
(deep link to German newspaper Tagesspiegel)