The volunteers fell asleep more quickly in 18-inch seats than in 17-inch ones, they also woke up less often and their legs twitched less.
Brainwave monitoring showed that deep, restorative sleep was especially sound.
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Overall, their quality of sleep was 50 per cent better.
Researcher Dr Irshaad Ebrahim said: ‘The difference was significant. All passengers experienced a deeper, less disturbed and longer night’s sleep in the 18-inch seat.
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