I've not read the article yet, but I'm going to assume it's affecting my sleep because I'm reading about how it's affecting my sleep at 04:21 instead of, you know, going to sleep. Just a guess
It's that, plus "notifications can disrupt your sleep."
“A much greater issue [than the blue light] is likely to be the content viewed,” says Peirson. “Reading work emails relating to impending deadlines is clearly going to cause anxiety, and anxiety is strongly related to insomnia.”