Ryan J. Walker et al., “Age, the Big Five, and Time-of-Day Preference: A Mediational Model,” Personality and Individual Differences 56 (2014): 170–174; Christoph Randler, “Morningness-Eveningness, Sleep-Wake Variables and Big Five Personality Factors,” Personality and Individual Differences 45, no. 2 (2008): 191–196.