All doctors, from radiologists to orthopedic surgeons, are told not to self-diagnose, and psychiatric conditions are the easiest to improperly self-diagnose, even for trained professionals (which most people on HN are not).
On top of that, ADHD is one of the most difficult conditions to properly diagnose. Unlike more overt disorders, like schizophrenia, the symptoms are less clear-cut, and unlike more visible disorders, like (true) OCD[0], the symptoms usually lie two or three layers deep, making it easy to confuse the cause of the symptoms if you're not careful.
There exist differential diagnoses for this purpose (disambiguating the cause of the symptoms); most people just don't have them done, because they're incredibly expensive (several thousands of dollars), as they're very time-intensive on the part of the doctor.
That said, the entire claim which precipitated this discussion - that ADHD is somehow related to sleep deprivation - is completely preposterous. ADHD is perhaps inappropriately named, yes, but that suggestion is like saying that the cure to depression is to "stop being sad".
[0] I say "true" to distinguish clinical obsessive-compulsive disorder from the casual use of the term (eg, "oh man, my OCD kicked in when I saw Comic Sans mixed with Papyrus").