August 2019 Issue
The Link Between ZZZs & Eats
By Jamie Santa Cruz
Vol. 21, No. 8, P. 32
When it’s time to hit the hay, the food you ate may determine whether you have a restless or a refreshing night’s sleep.
It’s no secret that Americans struggle with sleep. According to the National Sleep Foundation, between 50 million and 70 million adults have a sleep disorder.1 Approximately 30% of adults report symptoms of short-term insomnia, and approximately 10% have symptoms of chronic insomnia.2
As such, sleep improvement has become a massive industry, recently estimated to be worth upward of $30 billion.3 But as sleep-health companies peddle everything from mattresses and white-noise machines to high-tech pajamas in the pursuit of more shut-eye, some researchers are exploring a new potential avenue for improving sleep: food. An emerging body of research suggests there’s a relationship between sleep and diet quality, with the implication that improving our diet could result in better slumber.4,5
State of the Research
Much of what’s known so far about the connection between sleep and diet comes from cross-sectional studies, which measure both sleep and diet at a single point in time. According to Hassan Dashti, PhD, RD, a research fellow at the Center for Genomic Medicine at Massachusetts General Hospital, Harvard Medical School, this research suggests that diet is linked to both sleep duration and sleep quality, but it doesn’t indicate whether sleep patterns impact dietary habits or vice versa.
“These cross-sectional studies can’t distinguish the exact direction of the association between diet and sleep or determine whether a causal link even exists,” he says.
Current thinking is that the relationship between diet and sleep is reciprocal. Experimental data on sleep deprivation make clear that sleep patterns definitely influence eating habits, such as by increasing energy intake.6 This effect could be partly due to the fact that sleep deprivation alters levels of circulating hormones related to appetite control, increasing the ratio of ghrelin to leptin and thus enhancing appetite.7
Longitudinal and experimental data in the other direction—showing that diet impacts sleep—isn’t as abundant. But existing research does suggest that causality works in this direction as well, and that dietary patterns and intake of specific foods all can impact sleep health.4
This article reviews the key literature on various dietary patterns and specific dietary components and how they relate to sleep duration and quality.
Multiple cross-sectional studies have suggested that higher fat intake is linked to shorter sleep duration.7-10 A 2015 meta-analysis observed a significant association between shorter sleep duration and higher relative intake of saturated fat in particular, though the association was evident only in individuals younger than 65.11
However, the link between higher fat consumption and shorter sleep duration isn’t entirely consistent. A cross-sectional analysis of a subset of participants from the National Health and Nutrition Examination Survey (NHANES) 2007–2008 found that short sleepers (those who slept between five and six hours per night) had a higher intake of total fat than normal sleepers. However, very short sleepers (those who slept less than five hours per night) had lower intake of total fat than normal sleepers.12
While this research examined the relationship between fat intake and sleep duration, other research has suggested that fat intake may be related to sleep quality. For example, a 2015 analysis of middle-aged Finnish men by Tan and colleagues found that higher fat consumption was associated with insomnia.
Although few randomized controlled trials have examined the impact of diet on sleep, one noteworthy trial led by researchers at Columbia University found that saturated fat intake negatively impacted sleep quality. In the trial, which included 26 adults aged 30 to 45, participants consumed a controlled diet for four consecutive days, after which they were allowed to self-select their food on the fifth day. Greater intake of saturated fat on that final day of ad libitum selection predicted less slow-wave sleep.13
Like fat, protein intake has been linked to both sleep duration and sleep quality. In NHANES 2007–2008, very short sleep (less than five hours per night) was associated with lower total protein intake,12 and both short and long sleep were linked to a slightly lower percentage of energy coming from protein compared with normal sleepers.14 Among young adult women, those who reported sleep duration of less than six hours had lower energy intake from protein and higher energy intake from carbohydrates relative to normal sleepers.15
As for the association between protein intake and sleep quality, a cross-sectional analysis of 4,435 nonshift workers in Japan found that low protein intake (<16% of energy from protein) was linked to difficulty initiating sleep and poor quality of sleep. However, high protein intake isn’t necessarily desirable either. In the Japanese study, a diet containing >19% of energy from protein was associated with difficulty maintaining sleep.16
According to Essra Noorwali, MSc, PhD, a researcher in the Nutritional Epidemiology Research Group at England’s University of Leeds, further research is needed, but the effects of protein on sleep are “promising” and likely due to the amino acid tryptophan, which is a precursor to the sleep-regulating hormone melatonin.17
Epidemiologic research on the link between sleep and carbohydrate consumption has been conflicting. In several studies, low carbohydrate intake has been associated with insomnia, obstructive sleep apnea, and difficulty maintaining sleep.16,18 However, a cross-sectional study of middle-aged Japanese women found a significant trend toward poorer sleep quality with increased carbohydrate consumption.19
In the case of the Japanese study, the quality of carbohydrate appears to have been significant in mediating this association. High intake of confectionary and noodles was associated with poor sleep, and poor sleepers with the highest carbohydrate intake consumed more of these foods than did good sleepers with high carbohydrate intake.19
In regard to the connection between carbohydrate intake and sleep duration, some research has found an association between higher carbohydrate intake and shorter sleep duration in both younger and middle-aged adults.11,15 By contrast, in a sample of Chinese adults, sleep duration of less than seven hours was associated with a 1.8% lower carbohydrate intake compared with normal sleepers.10
Besides cross-sectional studies, several experimental studies have examined the relationship between carbohydrate intake and sleep quality. Multiple trials seem to suggest that dietary patterns lower in carbohydrates result in reduced REM sleep but increased slow-wave sleep.20-22
Carbohydrate intake also may affect sleep onset latency (ie, the time it takes to fall asleep).23,24 In a four-day controlled-feeding test of the effects of individual macronutrients, a high-carbohydrate diet decreased sleep onset latency as compared with high-protein, high-fat, and control diets.23
The impact of carbohydrates on sleep onset latency has been tied to the presence of the amino acid tryptophan.4 The data on carbohydrates and sleep is still limited, however, and none of the existing randomized controlled studies have examined long-term effects.
The few studies that have examined the association between fiber and sleep duration collectively suggest that higher fiber intake is associated with normal (as opposed to long or short) sleep duration.
In NHANES 2007–2008, normal sleepers had the highest fiber intake, whereas short and long sleepers (those who slept five to six hours or more than nine hours) both had lower fiber intake, and very short sleepers (those who slept less than five hours per night) had the lowest fiber intake.12
Cross-sectional studies of other populations have produced similar findings.15,25 Experimental evidence is scant, but the aforementioned trial conducted at Columbia University found that higher fiber intake resulted in deeper, more restorative sleep, as indicated by increased slow-wave sleep.13
Data on the association of sugar with sleep duration are limited and conflicting. In NHANES 2005–2010, higher total sugar intake was associated with short sleep (equal to or less than six hours per night).14 Consistent with this finding, the NHANES 2007–2008 analysis found that short sleepers (five to six hours per night) had higher sugar intake than normal sleepers, but counterintuitively, very short sleepers (less than five hours per night) had lower sugar intake than normal sleepers.12 Therefore, it’s unclear what impact, if any, sugar may have on sleep duration.
With respect to the relationship between sugar and sleep quality, the Columbia University trial found that higher sugar intake negatively impacted sleep quality, in that it was associated with more sleep arousals during the night.13
Fruits and Vegetables
One recent prospective analysis of nearly 14,000 women in the United Kingdom (UK) Women’s Cohort Study found, contrary to the authors’ expectations, that every additional portion of total fruit and vegetable consumption was associated with shorter sleep, albeit with a small effect size.26
However, other cross-sectional and prospective studies of nationally representative samples of UK adults have found that sleep duration is nonlinearly associated with fruit and vegetable consumption and that participants sleeping the recommended seven to nine hours per night have the highest intake compared with both short and long sleepers.27,28 Several cross-sectional studies of other populations likewise have found that short sleep duration is associated with lower consumption of fruits and/or vegetables.15,29,30
Several small, short-term experimental studies have focused on the beneficial effects of certain specific fruits, such as tart cherries. In healthy young adults, a supplement of tart cherry juice increased urinary melatonin concentration, total sleep time, and sleep efficiency compared with placebo.31 In adults with chronic insomnia, a similar supplement significantly reduced insomnia severity and wake time after sleep onset.32
A separate intervention tested the effects of seven different cultivars of Jerte Valley cherries. All seven cultivars produced an increase in urinary melatonin and total sleep time for both middle-aged and elderly participants. Certain cultivars also produced other sleep benefits, including decreased nighttime awakenings and reduced sleep onset latency, but these benefits varied depending on the age group (middle-aged vs elderly).33
The positive impact of cherries, if confirmed, is likely related to their relatively high concentration of melatonin and their high antioxidant capacity.4 Tart cherry juice is known to reduce circulating inflammatory markers and increase antioxidant capacity in athletes.34,35 Because patients with sleep disorders tend to demonstrate high levels of oxidative stress, the antioxidant properties of cherries may help promote sleep by minimizing oxidative damage.36
In addition, kiwifruit has been suspected of having a positive impact on sleep duration and quality. In one four-week trial of adults with sleep problems, consumption of two kiwifruits per day one hour before bedtime significantly increased total sleep time and sleep efficiency, while also decreasing sleep onset latency and wake time after sleep onset.37
The benefits of kiwifruit have been attributed to its high antioxidant capacity and its unusually high content of serotonin, which may promote sleep.4
Kiwifruit also is a good source of folate. Whereas folate deficiency has been linked to insomnia and restless leg syndrome, folate supplementation has been shown to alleviate these symptoms.38,39 Again, however, larger trials are needed to confirm kiwifruit’s sleep benefits.
Repeated studies have shown a correlation between the Mediterranean diet and healthy sleep. For example, in the Multi-Ethnic Study of Atherosclerosis ancillary sleep study, individuals with higher adherence to a Mediterranean diet were at lower risk of insomnia symptoms and of insomnia symptoms in combination with short sleep duration.40
Analyses of French, Spanish, and Greek adults likewise found that adherence to a Mediterranean-style diet was associated with reduced insomnia symptoms or overall improved sleep quality.41-43 As for the connection between the Mediterranean diet and sleep duration, the trend appears to be that higher adherence to a Mediterranean diet is associated with a greater likelihood of normal sleep duration.40,42
Current research on the Mediterranean diet and sleep can demonstrate only associations, not causality. However, there’s some evidence to suggest that the high fiber and low saturated fat content of the Mediterranean diet may be responsible for the observed links. In the Columbia University randomized crossover trial, each 10 g increase in fiber consumption was associated with a 2.6% increase in time spent in slow-wave sleep, whereas each 10% increase in energy from saturated fat was associated with a 7.1% reduction in time spent in slow-wave sleep.13
Esther Lopez-Garcia, PhD, MPH, MhPharm, a professor of preventive medicine and public health in the School of Medicine at Spain’s Universidad Autónoma de Madrid, suggests that other biological mechanisms also may be at work. “For example, the anti-inflammatory and antioxidant effect of the [Mediterranean diet] may favor healthy sleep, or the consumption of olives and grapes linked to this diet, which are rich in melatonin, a compound that favors the circadian cycle [may do the same],” she says.
Finally, Lopez-Garcia notes that “adherence to healthful diets also implies cooking raw products instead of processed, devoting time to eating and planning eating, which is associated with a more organized and less stressful life,” and this, in turn, “may have an important impact on sleep patterns.”
According to Dashti, the next step is to move from cross-sectional research toward more longitudinal and experimental study designs, which will give researchers a better idea of how particular dietary habits affect sleep patterns over time. In addition, there’s a need for more objective assessment of both diet and sleep to increase the reliability of research findings.
“Previously we have relied on self-reported data on sleep habits and food intake, and even though people have some idea of what they’re eating and how much they’re sleeping, it’s not that accurate,” Dashti says, adding that technological advancements—such as wearable sleep monitors—are making it easier to gather accurate data. “We’re better able to assess sleep duration, and also other dimensions of sleep such as quality.”
Another important step for researchers will be to examine how the timing of consumption affects the impact of specific foods. “What food, at what quantity, at what time?” asks Marie Pierre St-Onge, PhD, an associate professor of nutritional medicine at New York’s Columbia University Irving Medical Center. “Should you eat the food in the morning or before going to bed?”
As the research unfolds, St-Onge envisions that dietitians could have an expanded role in treating individuals who suffer from inadequate sleep. “Those patients with insomnia or obstructive sleep apnea—we could have dietitians be involved in their treatment showing that diet can influence their sleep at night,” she says.
Takeaways for Dietitians
Because research surrounding dietary impacts on sleep is still in the early stages, there are few specific recommendations dietitians can make to clients regarding what foods to eat for better sleep. For now, a key message to convey is that there’s indeed a connection between diet and sleep.
“Overall, what the studies seem to suggest is that to improve your sleep, you need a better overall diet,” Dashti says.
Some of the most consistent findings so far relate to the Mediterranean diet pattern. “The more ‘Mediterranean’ your dietary habits are, the better your sleep quality is,” says Mary Yannakoulia, PhD, an associate professor of nutrition and eating behavior at Harokopio University in Athens, Greece.
Although the mechanisms are unclear, Yannakoulia encourages this dietary pattern for anyone looking to improve sleep. She asserts, however, that the benefits of a Mediterranean diet may not come from food alone. “The Mediterranean diet is part of the Mediterranean lifestyle that includes, apart from the diet per se, adequate rest, regular physical activity, and conviviality,” she says.
While also a proponent of the Mediterranean diet, Lopez-Garcia encourages dietitians to recommend other healthful dietary patterns as well, such as the Dietary Approaches to Stop Hypertension, the Healthy Eating Index, and plant-based diets in general.
“They all advocate for using minimally processed foods and high-quality fats, with low amounts of added sugars and salt,” she says. “In addition to the important nutrients they provide, these diets are digested slowly, which leads to a gentle rise in blood sugar and insulin. This produces fewer peaks of hunger, less deprivation, and less production of stress hormones. All of this also has a beneficial impact on sleep.”
Finally, dietitians should make sure to discuss the impact of stimulants with clients. “The circadian system is intertwined with sleep, and thus nutritional compounds that affect the circadian system influence sleep,” Noorwali says, adding that the most robust effects are seen with caffeine and alcohol.44
A wide array of study populations have found that daily caffeine intake is strongly associated with sleep problems—even a small amount of regular caffeine consumption is associated with altered sleep duration, onset, and perceived quality.45 Similarly, while alcohol consumption before sleep makes it easier to fall asleep quickly, alcohol alters sleep architecture and is associated with disrupted, poor-quality sleep.46 “Advice on moderation and timing of these compounds will be beneficial,” Noorwali adds.
— Jamie Santa Cruz is a freelance writer of health and medical topics based in Parker, Colorado.
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