I know, but I've headaches since I was about 10 (now almost ~30), and the only causes for this that I can seriously come up with is: being sensitive af in general, which then leads to being (subconsciously) easily stressed, which then, results in headache. This happens during the day, but I also (more) frequently wake up with a headache for no reason. This even happens in the summer, when I'm not doing ANYthing (no school, university, work). I am generally perfectly healthy. I am hydrated, eat healthy, etc. I don't exercise an awful lot, but when I do (for months), I barely notice any difference.
Being "sensitive" (whatever that means exactly) is the only weird but plausible explanation. Reading the other comments, though, like sleep apnea, I'm curious if that's anything...