... for youth in Iceland aged 13 to 15 years in February 2016, February 2018, and November 2023.

Apparently they did not control for medication or supplements, such as vitamin D treatments, artificial UV light exposure, or antidepressants.

Alternate interpretation: when it's dark for half the year your mental health isn't dominated by phone access. School's probably a social, cognitive and energy highlight in the otherwise permadark day.