[lm28469]

Did we prove that early birds vs night owls exists on a biological level ?

As in, after taking into account individual habits / conditions: coffee, alcohol consumption, quality of sleep (noise/light in the room), physical activity, stress level, usage of artificial light before bed, &c.

If I strap a 10000 lumen blue light in front of your face every single night until 2 AM for a few weeks I guarantee you'll turn into a night owl by the article definition (shifted melatonin and cortisol production cycles).

They kind of make this point themselves: "genes can even shift a person’s natural waking time by up to 25 minutes." and in the linked article "the mean sleep timing of the 5% of individuals carrying the most morningness alleles is 25 min earlier than the 5% carrying the fewest.".

If genes only account for up to 25 min then the rest might come to personal routines and behaviours.