Population chronotypes are roughly distributed as one-third morning types, one-third evening types, and one-third intermediate types.
"I think the data are that it's kind of roughly a third, a third, a third-ish: a third of people are morning people, a third-ish of people are evening people, and then a third-ish are somewhere in the middle." (said at 0:47:06)
Population chronotypes are continuously distributed along a normal (near-Gaussian) curve rather than divided into equal thirds. Consequently, intermediate (neither morning nor evening) types make up the largest proportion of the population—typically around 50% or more depending on the scoring criteria and age group—while clear morning and evening types represent smaller proportions (often ~10–25% each under modernized cutoffs, or heavily skewed toward morning types with age under traditional Horne-Östberg criteria).
- partial: The epidemiology of morningness/eveningness: influence of age, gender, ethnicity, and soci… (Journal of biological rhythms 2006)
"According to the Horne and Ostberg classification, 49.8% of the total population was classified as morning type compared to 5.6% having an evening-type preference. However, using new cutoffs for middle-aged working adults described by Taillard et al. (2004), 24.7% of the population was morning type and 26.4% was evening type." (abstract, results, passage verified)
pubmedfull study (doi) - context: Epidemiology of the human circadian clock. (Sleep medicine reviews 2007)
"Sleep and wake times show a near-Gaussian distribution in a given population, with extreme early types waking up when extreme late types fall asleep." (abstract, passage verified)
pubmedfull study (doi)