It depends on what treatment you are referring to....
Wear what you feel comfortable in. Temperature in the mine is supposed to be 74 though it felt cooler last time I went than before. Also they have blankets you can use if you need it on the back of your chair
Wear something comfortable, they have blankets if you get chilly
Wear something warm if your the type to get cold because it can get a bit too cool for some
Regular clothes
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