If there is one place you would have thought a sleep-deprived person might be able to stop herself dropping off, it’s in a lecture about sleep. But that’s what this atmospheric but somewhat heavy-handed debut feature from Norway has its protagonist Mona (Eili Harboe) do as she is introduced by dishevelled academic Aksel (Dennis Storhøi) to the possibility that she has become the victim of the mythical incubus Mare. This may explain a recent run of freakish dreams in which she’s tormented by a vampiric doppelganger of her caring boyfriend Robby (Herman Tømmeraas). Nightmare also belongs to the school of property horror already occupied by The Tenant and Mother! Left alone by Robby, a high-flyer preoccupied with some kind of algorithmic investment venture, Mona is charged with renovating their sprawling new apartment which they acquired on the cheap after its previous occupant, who was pregnant, died in a mysterious accident. Their neighbours, who have a newborn baby and are prone to staring eerily across the courtyard, seem to have issues, too. But none of this rings any alarm bells until Mona – vaguely thinking about having kids with Robby – begins sleepwalking. Restricting Mona’s mental disintegration in the dingy apartment, writer-director Kjersti Helen Rasmussen at first confines her film to the tasteful realist parameters of much “elevated horror”: that the encroaching supernatural is somehow a Babadook-style manifestation of Mona’s desire to resist adulthood, responsibility and maturity. There is nothing wrong per se with the second-half switch as the Mare is revealed as something more, but the film doesn’t flesh things out convincingly. There’s little in the way of a backstory for the demon and its designs, and no explanation for why Aksel – a sleep-therapy version of Halloween’s Dr Loomis – is so invested in such patently unscientific notions. Nightmare does maintain a crepuscular glower throughout, and Rasmussen chops up waking and dream scenes to disorientating effect; sometimes we’re not sure which is which. But with several rather forced plot points also intruding into this delirium – like their neighbour’s final skull-pulverising meltdown – the film flirts with drifting into nonsense; less inspired dream logic than a retreat from its initial psychological clarity. Nightmare is available on Shudder on 29 September.