> I would have been very surprised to have seen a difference in cognitive ability this quickly

Sleep deprivation is really, really bad for you. Here [0] is one example that tries to measure reaction times compared to drinking (in the context of driving). Being tired is pretty much being drunk. Here [1] is another on cognitive activity.

It’s not surprising to me in the slightest (anecdotally, I suffer from bouts of insomnia and my behaviour, mood and cognitive performance in work is definitely lower during those times. Even a single nights sleep shows a huge change in my mood INE) that if reducing smartphone usage they get more sleep that they pretty much immediately saw improvements.

[0] https://pubmed.ncbi.nlm.nih.gov/32571274/ [1] https://pubmed.ncbi.nlm.nih.gov/23029352/