https://www.sciencedaily.com/releases/2009/06/090623150621.htm https://www.sciencedaily.com/releases/2009/06/090623150621.h... https://www.livescience.com/5362

https://www.sciencedaily.com/releases/2009/06/090623150621.htm https://www.sciencedaily.com/releases/2009/06/090623150621.h... https://www.livescience.com/53624-morning-person-genetic-influence.html https://www.livescience.com/53624-morning-person-genetic-inf... https://www.sciencedirect.com/science/article/abs/pii/S0065266017300202 https://www.sciencedirect.com/science/article/abs/pii/S00652... I didn’t read all of these (and their references) in great detail, but there seems to be studies suggesting that chronotype is genetic. Some studies seem to be more reliable than others (ie as you said, some seem to rely on self reporting, but others seem to rely more on genetic testing or other things). But this isn’t even close to my area of expertise so take with a grain of salt and read up on it yourself.