I'm a 20 year old male and I have been severely underweight since early puberty. Currently at 16.1 BMI.
I also get tiredness attacks during the day even if I had a good night's sleep (I usually sleep for about 8-9 hours a night). Interestingly enough, reducing my nightly sleep to 5-6 hours actually diminishes this effect, but I can't maintain that sleep cycle because after 2-3 nights I oversleep alarm clocks.
I am diagnosed with persistent depressive disorder (PDD) but not currently taking any antidepressants.
I have done the usual blood tests for thyroid hyperfunction or liver problems / cirrhosis. All negative.
I do not have an eating disorder and my calorie intake is slightly above normal.
Most people, including doctors, tell me it's probably a genetic thing, but while both my parents are definitely on the thinner side, I can't quite believe that's all there's to it. My brother has normal weight.