Strange conclusion, with an obvious flaw:
Such work may have implications for the management of sleep disturbance if fear of the dark is a reliable observation in poor sleepers.
At issue is the line ahead in the same P:
the design of this study precludes any causal statements, so it may be that feeling uncomfortable in the dark predisposes people to poor sleep or it could be that protracted poor sleep creates a negative association with the dark.
The body, brain and mind are known to be negative feedback engines, think equilibrium reactions with variable midpoints. I am thinking specifically of the recent decade plus of work on glial cells mechanisms for brain interaction, but similar functions act as methods of moving the equilibrium point, much like a sea anchor. As such, it is seen frequently that a cause can become an effect as the equilibrium process instantiates the no longer new norm. All this really says is we are adaptive.
Poor sleepers can become conditioned to be nyctophobic by exposure. Nyctophobes will avoid being in the dark, and the human design tends to avoid sleeping during light time hours. Exposure therapy, once the underlying cause is treated is frequently utilized to move the afore mentioned balance point. So, sleeping in the dark can cause nyctophobia in people without it if you startle the subject sufficiently, yet once the startle is removed exposure reverses the process. So the are correct, while there is correlation, causation cannot be determined by a study that really says: Fear is a secondary response typically, put someone in a fearful situation and the will begin to avoid that situation.
Anecdotally I know plenty of people with fear issues. Many of the same set also have sleep issues. OSA, for example, will exacerbate many issues; but, except in the rare cases where OSA actually is the source of the fear (waking, startle, etc) , it is a factor in the response, not the cause.
Macbeth Act 5, scene 5, 23–28