Complex sentence: Beginning in his early life, Lovecraft is believed to have suffered from sleep paralysis, a form of parasomnia; he believed himself to be assaulted at night by horrific "night gaunts".
Split into simpler sentences: Beginning in his early life, Lovecraft is believed to have suffered from sleep paralysis (a form of parasomnia). He believed himself to be assaulted at night by horrific "night gaunts".