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.
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.