Would it be a gas leak? I'm not sure about all types of gas leaks, but I know that at least when it comes to carbon monoxide leaks, those tend to lead to blank spots in the memory, which Yuna hasn't been displaying thus far. I'd also expect to see a gas leak affecting Rena, which it doesn't seem to be.
I'm wondering if perhaps Yuna is simply not falling asleep at night due to stress over Rena's home salon which is leading to her passing out during the day. It would be a little anti-climactic but I'm not sure I really see this story going into particularly dark territory like health complications from gas leaks, plus we've already seen Yuna stressing a lot over the salon, so it being lack of sleep over stress would tie in to that.