I would like to turn on mood lighting automatically when I'm taking a shower, what's the best way to detect that?

• I've considered humidity sensors, but that will take a while to kick in and not immediately when the shower starts.
• I've also considered a water leak sensor but those are not meant to be trigger every day for a long time like 10-15 minutes. That would kill the battery, right?
• Smart valves can kind of work, but it feels like overkill because I don't care about the valve functionality.

I would prefer the solution to be battery powered (don't want to be messing with too many cables around a shower), but I can't seem to find one.