For the pure fun of breaking the narrative I found original article, it's here: https://bmjpublichealth.bmj.com/content/2/1/e001000

Time of day (or time after waking up per subject) when tests were administered has not been controlled. Cognitive abilities are mediated by wakefulness (not to mention, related for most people, digestive processes) cycle.

If '"Night owls" smarter than morning people' sounds more plausible than 'time since waking up and last meal predictive of cognitive performance' it's time to get one's identity checked. And I can't imagine 'journalists' from thrash like Sky (Guardian this time) not knowing that, which brings me to the final point: what is this link doing here?