The problem with advice about how diet affects your sleep

What about the reverse – how does diet affect our sleep quality and quantity? It’s a question pondered by many people the morning after another fitful night’s sleep. Did I eat something that disrupted my sleep? Is there something I should be eating to improve my sleep? Will a glass of Horlicks or some tart cherry juice help me sleep better? And is the Mediterranean diet the answer to my prayers?

Greek researchers recently looked at links between the predominantly plant-based diet – it is heavy in fruit, vegetables, legumes, nuts, olive oil and whole grains, includes moderate amounts of fish and poultry, and is low in dairy products, red meat, processed meats and sweets – and the sleep quality of older adults. Using self-reported questionnaires for sleep and food intake, they found people aged from 65 to 75 who stuck closely to the diet had better sleep quality, but not necessarily sleep duration. No relationship was found between the Mediterranean diet and sleep among people older than 75.

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However, given that this is an observational study, it doesn’t prove the diet caused the better sleep quality, although it adds to the growing evidence linking the two. For example, researchers found female Japanese workers who ate more confectionery and noodles had poorer sleep quality, whereas those who ate more fish and vegetables slept better.

Experimental studies have investigated whether eating more or less of the major macronutrients in our diet – carbohydrates, protein and fat – affects sleep quality. One study found a high-protein diet decreased the number of waking episodes compared with a control diet, whereas high-fat and high-carbohydrate diets had varying effects on overall sleep quality.

And researchers have also looked at individual foods to investigate whether they can aid sleeping. In the 1970s, a number of clinical trials found that a warm milk drink with Horlicks powder 30 minutes before bedtime resulted in fewer small movements during sleep and increased total sleep time among older adults.

Kiwifruit, too, has been linked to improved sleep. A Taiwanese study noted two of the fruit eaten an hour before bed significantly increased total sleep time and restfulness.

Tart cherry-juice, drunk in the morning and evening, reportedly also reduces insomnia. In a group of younger adults, a week of tart cherry-juice supplementation resulted in greater total sleep time and restfulness compared with a placebo juice.

This all sounds positive, except for one thing: the studies concerned were funded by the product manufacturers – Horlicks, Zespri International and a tart cherry-juice producer respectively. In an ideal world, that wouldn’t make any difference, but history (and statistics) tell us that industry-funded research is more likely to produce positive results that favour the funder.

So, what do we know for sure about diet and sleep? Not a lot. But the evidence does tend to lean in the same direction as other healthful-living advice: that is, if you eat more wholefoods, such as unprocessed fruits, vegetables and whole grains, and fewer processed foods, particularly sweets and sugar-sweetened drinks, your chances of a better night’s sleep might improve.

You could, of course, try a tin of Horlicks, some kiwifruit or tart cherry juice – but buyer beware.

This article was first published in the December 1, 2018 issue of the New Zealand Listener.