Why can't I sleep?

I'm 19, and for the past few years I've been able to live off of a minimal amount of sleep each night. Regularly I'll get somewhere between 2-4 hours a sleep, and about a couple times a month I'll have one good night where I'll sleep 7+ hours.

I'm not fatigued during the day, I'm not stressed or depressed or even angry about anything, I'm quite laid back. The reason why i don't figure its Insomnia is because all of the research I've looked up, points out that you'll feel tired during the day, to which I rarely am.

I haven't seen a doctor about this whatsoever, I have an appointment booked for January 6. I'm just impatient and wondering what's Reddit's opinion on my sleeping habits. Is there a definition for something like this? I haven't been able to find anything related to this issue, Insomnia is the closest but I don't feel its related entirely.