Do poor sleepers have a fear of the dark?

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By Dr. Simon Kyle

Models of insomnia suggest an important role for conditioned arousal in the maintenance of poor sleep. That is, repeated pairing of bedtime with arousal and negative emotion may mean that, over time, the simple act of entering the bedroom may stimulate wakefulness. Researchers from Ryerson University recently asked the question of whether the presence of darkness may also induce greater physiological reactivity and subjective discomfort in poor sleepers, relative to good sleepers.

The research team, led by Colleen Carney, recruited 20 good sleepers and 26 poor sleepers. Participants came to the sleep laboratory during daylight hours and completed 1) a questionnaire relating to discomfort associated with darkness; and 2) a “startle paradigm” to index physiological reactivity. This startle paradigm involved the participant lying down on the bed, listening to bursts of sudden white noise (delivered through headphones) during both light and dark conditions. Electrodes were placed proximal to the eye in order to detect muscle changes associated with eye movements. Previous work has indicated that sudden bursts of noise (the startle stimulus) result in eyeblinks (the startle response) and that latency to eyeblink tends to be reduced (greater startle response) during dark conditions relative to light conditions.

In order to test whether poor sleepers have a reliable physiological fear of the dark, researchers hypothesized that, across several blocks of sudden bursts of noise during darkness, good sleepers would evidence habituation (i.e. less physiological startle response on second presentation). However, for poor sleepers, it was predicted that this group would not evidence habituation to the same degree, indicating a stable darkness-related fear.

The research team found that poor sleepers did in fact report greater levels of darkness-related discomfort through questionnaire report. Moreover, while good sleepers showed increased eyeblink latency (reduced startle response) across trials for the dark phase, this was not found for poor sleepers. The authors discuss the possible implications of their work: “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. If someone has a fear of the dark, they are at greater risk for sleep disturbance because of the increased arousal associated with the lights being turned-off.” Such work may have implications for the management of sleep disturbance if fear of the dark is a reliable observation in poor sleepers.

Carney, C.E., Moss, T.G., Atwood, M.E., Crowe, B.M., Andrews, A.J. (2013). Are poor sleepers afraid of the dark? A preliminary investigation. Journal of Experimental Psychopathology, In Press.

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