I hadn't heard the term, hypnagogia, or the effects it encompasses, so I'd be baffled. And even those readers who know the word or the condition may not realise that this character experiences the effects and, in particular, not how frequently (to the extent that it's the expected cause of any unusual images or sounds) unless you tell, or have previously shown, them that. Otherwise your explanation, "it's like the time..." will leave them none the wiser (because that time may have been a spooky or unsettling experience).