Circadian rhythmic expression differs among individuals. Some individuals are night people and some are morning people. Mechanisms and evolutionary rationale for such differences are unknown, as is the reason behind related disorders. Studies have shown that “eveningness” may be associated with mood disorders, ADHD, and eating disorders. (pdf)
Circadian Typology could be related to kin selection, via Hamilton’s Rule
Typology calls into question the premise of First Hours, Best Hours.