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Health Series: Research Spotlight

Why we go transiently ‘blind’ when sleeping with a smartphone

Research raises awareness of the phenomenon for the public and physicians

by George Wigmore (Senior Communications Officer)

Spending too much time using smartphones in bed could cause temporary blindness, according to a new study by academics from City University London, Moorfields Eye Hospital, King’s College London, and the National Hospital for Neurology and Neurosurgery.

While the experience is completely harmless, the authors aimed to highlight the phenomenon to raise awareness for physicians and reduce costly investigations, while also reassuring patients. The paper is published in the New England Journal of Medicine.

Describing two women who presented with visual problems in one eye, the team found that when the patients viewed their smartphone in bed the symptoms seen were due to their eyes adjusting differently to the light emitted by the device. As while the viewing eye became light adapted, the eye blocked by the pillow was dark adapted. As a result, when both eyes were uncovered in the dark, the light-adapted eye was perceived to be ‘blind’, with the effect – which is known as the differential bleaching of photo pigments - lasting a few minutes.

Although not confined to smartphone use, due to wide usage of such devises the effect is most commonly observed with them. The effect has also been observed in the past, as many years ago a similar phenomenon known as ‘Carsonogenic blindness’ – taking its name from the Johnny Carson show - was well known in the Neuro-Ophthalmology community due to the habit of watching TV in bed with one eye covered.

However, smartphone use is likely to result in this type of phenomenon becoming more common. The effect is also reported when patients with cataracts that affect mostly the centre of the lens in one eye view bright lights with both eyes (and not just smartphones). The differential bleaching of photo pigments in the two eye causes relative loss of vision when turning towards darker areas with the non-cataractous eye showing the greatest loss of sensitivity to light.

To further investigate the effect, the authors then viewed a smartphone screen at arm’s length and quantified the time course of recovery of sensitivity in the dark using a rapid dark adaptation test developed at City University London. They found that visual sensitivity of the eye which had not been covered and had been viewing the smartphone was significantly reduced, taking several minutes to recover, and this reduction in sensitivity was measurable at the level of the retina.

John Barbur, Professor of Optics & Visual Science at City University London and co-author of the paper, said:

“Although most people view screens binocularly with both eyes open at the same time, people frequently use smartphones while lying down, when it may be more comfortable to view smartphone screens with one eye or one eye may be inadvertently covered. Smartphones are now used nearly around the clock, and manufacturers are producing screens with increased brightness to offset background ambient lighting and thereby allow easy reading.

“As a result, as smartphones become more popular and pervasive presentations such as we describe are likely to become more frequent. Our cases show that detailed history taking and an understanding of retinal physiology can reassure both patient and doctor and can avoid unnecessary anxiety and costly investigations.”

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