A team from Harvard Medical School have found that it takes longer to fall asleep after reading from a light-emitting e-reader, compared to a printed book, leading to a poorer quality sleep and increased tiredness the following day.
Our bodies are kept to schedule by an internal body clock, which uses light to tell the time. Light-emitting devices, such as e-readers, smart phones and tablets, produce blue light which our body clocks perceive as daylight.
When our bodies are subjected to light, our alertness levels increase and therefore falling asleep becomes more difficult.
As part of the research, twelve people were locked in a sleep laboratory for two weeks. They spent five days reading from a paperback and five days reading from an iPad.
Researchers took regular blood samples, finding the e-reader users to have a reduced amount of the sleep-promoting hormone melatonin.
People using e-readers took longer to fall asleep, causing a delay in their body clock; this reduced the amount of time spent in ‘deep sleep’ and reduced alertness the following morning.
The findings were published in the journal Proceedings of the National Academy of Sciences.