Can I bring my phone in a sauna is a reasonable question with an unambiguous answer: no, and the reason is the battery rather than the humidity. Phone makers publish operating ranges that a sauna exceeds by a wide margin, and the damage from repeated exposure is cumulative and not covered by a warranty.
What the heat does
Lithium batteries degrade faster at high temperature, permanently and cumulatively, so a phone that survives one session is still losing capacity. Adhesives that hold the screen and back soften. Displays can show blotching. Most phones will also throttle or shut down to protect themselves, which is the polite version of the same problem.
What the makers say
Manufacturers publish an operating temperature range for their handsets, and the upper limit is far below a sauna's air temperature, along with a storage range that a sauna also exceeds. Heat damage is generally excluded from warranty cover, so this is not a risk the maker is carrying on your behalf.
It is not the water resistance
A phone rated for immersion is rated for cool water, and that rating says nothing about steam. Hot vapour drives moisture past seals designed for liquid, and the pressure changes as the phone heats and cools pull it further in. Water resistance is exactly the wrong thing to rely on here.
What to use instead
A cheap analogue sauna timer and a wall thermometer cover the two things people actually take a phone in for. For music, a speaker outside the door with the volume up works better than anything inside, because heat rated speakers are rare and expensive. If you want the phone for safety, leave it just outside within reach.
Questions people ask about can i bring my phone in a sauna
Will one session damage a phone?
Possibly not visibly, but battery degradation from heat is cumulative and permanent. Repeated sessions will shorten its life.
What about a waterproof phone?
Water resistance ratings are for cool water, not hot vapour. Steam gets past seals that liquid water would not.
How should I time a session then?
A wall mounted analogue sauna timer. It is cheap, it needs no battery and it is the standard fitting in a cabin for good reason.