Insomnia is a common sleep disorder. With insomnia, you may have trouble falling asleep, staying asleep, or getting good quality sleep.

and it was absolute hell, at least for Izuku.

Izuku was currently struggling to fall asleep— It was like 0:00 in the night,for God's sake. He had no idea on what to do about it. Meanwhile, Katsuki was cuddling him,kissing him senselessly around his face.