If you curl up under the duvet with an e-book for a bedtime read then you are damaging your sleep and maybe your health, US doctors have warned.
A team from Harvard Medical School compared reading paper books and light-emitting e-readers before sleep.
They found it took longer to nod off with a back-lit e-reader, which led to poorer quality sleep and being more tired the next morning.
Original Kindle readers do not emit light so should be fine, say experts.
Experts said people should minimize light-exposure in the evening.
Whether you are perusing the Man Booker shortlist or leafing through Zoella, the impact of reading on your sleep is probably the last thing on your mind.
But there has been growing concern about the dangers of light before bedtime.
Body clock
Our bodies are kept in tune with the rhythm of day and night by an internal body clock, which uses light to tell the time.
But blue light, the wavelength common in smartphones, tablets and LED lighting, is able to disrupt the body clock.
Blue light in the evening can slow or prevent the production of the sleep hormone melatonin.
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