There are many studies.[1] Large-scale genetic studies have found markers associated with self-reported "morningness".[2][3] At least some of these markers are

There are many studies.[1] Large-scale genetic studies have found markers associated with self-reported "morningness".[2][3] At least some of these markers are near genes known to affect circadian rhythms. Observational studies control factors like caffeine intake and light exposure. [1] https://en.wikipedia.org/wiki/Chronotype https://en.wikipedia.org/wiki/Chronotype [2] https://www.nature.com/articles/ncomms10448 https://www.nature.com/articles/ncomms10448 [3] https://journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.1006125 https://journals.plos.org/plosgenetics/article?id=10.1371/jo...