In this case the correlation is due to a causal relationship between genetic variation and chronotype that is well established. The roles of many of the genes involved have been established in lab organisms -- fruit flys and mice mostly -- yes you can observe and manipulate chronotype in flys. The same circadian rhythm genes have similar functions in people. There are rare mutations in these genes that cause familial sleep disruption syndromes, as well as common variants that have small effects that can be detected in large studies.
