Sleep in teenagers can be improved by just one week of limiting their evening exposure to light-emitting screens on phones, tablets and computers. The study indicates that by simply limiting their exposure to blue-light emitting devices in the evening, adolescents can improve their sleep quality and reduce symptoms of fatigue, lack of concentration and bad mood, after just one week. Those who had more than 4 hours per day of screen time had on average 30 minutes later sleep onset and wake up...

Might be worth investigating what else the teens are doing in their newly found 4 hours of time not sitting in front of a screen. If they are exercising, that might be the reason they are sleeping better, rather than blue light avoidance.