In 1,936 adults from Catania, those eating closest to the 'planet-friendly' diet were more likely to say they slept well. They didn't sleep any longer, and the answers were collected in 2014

The Planetary Health Diet is the EAT-Lancet Commission's sketch of a way of eating that could feed ten billion people without wrecking the planet: plenty of vegetables, fruit, whole grains, beans and nuts, modest fish, dairy and eggs, and only a little red meat and added sugar. Researchers keep asking whether it is also good for the people eating it. A team led from the University of Catania asked about sleep, and published the answer in Nutrients on 29 October.

They went back to the MEAL study, which in 2014 and 2015 randomly picked adults from family doctors' lists in the city of Catania, in Sicily. Of 2,405 people invited, 2,044 took part, and 1,936 gave usable diet records. Each filled in a 110-item questionnaire on what they had eaten over the previous six months, and the team turned it into a Planetary Health Diet Index score out of 150. The average was 66.5. They also answered the Pittsburgh Sleep Quality Index, a widely used 19-question survey; a total under 5 counts as adequate sleep quality.

In the quarter of people with the lowest diet scores, 60.8% reported adequate sleep. In the top quarter it was 71.3%. After allowing for age, sex, body mass index, physical activity, education, job, smoking and calories, the top quarter had 1.65 times the odds of adequate sleep (95% CI 1.20 to 2.28). The second quarter came out ahead too (1.47), but the third quarter's result could have been chance (1.35, CI 0.98 to 1.85), so the pattern is a step up from the bottom rather than a smooth slope. The most consistent link was with how well people rated their own sleep, which was better in all three upper quarters. How long they slept, how efficiently, and whether they took sleeping pills showed no link at all.

Split by weight, the link sat mostly with people who were overweight or obese: there, the top quarter had 2.40 times the odds of adequate sleep (1.46 to 3.95), while among normal-weight people no quarter differed from the bottom one. The authors are careful here. They did not run the formal test for whether the two groups really differ, undiagnosed sleep apnoea is more common with higher weight and was not measured, and an earlier paper on the same Sicilian volunteers, about the Mediterranean diet, found the opposite split, with links in normal-weight and overweight people but not in those with obesity.

So what does it show? In one Sicilian city a decade ago, people who ate more like the planetary reference diet also reported somewhat better sleep. It cannot say which came first: diet and sleep pull on each other in both directions, as the authors point out, and a careful eater may simply lead a calmer, more health-minded life. Caffeine, alcohol, mood, stress and shift work were not in the model. Diet and sleep were both self-reported, and there was no sleep tracker or sleep study. The authors call it hypothesis-generating and ask for trials. Eating more beans, vegetables and nuts has plenty of reasons behind it already. Better sleep is, for now, a maybe on that list.