When I was in my early 20s, I weighed under 60kg (which put me at a BMI of 19ish). I was tired all the time. I went through the usual gauntlet of tests, including a sleep study. The doctors couldn't find any explanation, and after months of this I sort of got used to it.

The months turned into years, and in my mid 30s, my upstairs neighbour complained about my loud snoring. Back to tests, another sleep study. Diagnosis: sleep apnea. They then asked me why I never got treatment in the first place, because apparently that first sleep study indicated sleep apnea ... but because the AHI was on the lower end, and because I was young and skinny, the doctor didn't think it was relevant for me to know.

Sigh.