Beyond its role in encouraging a good mood and bolstering productivity, sleep is essential to the maintenance of good physical health and wellbeing.1 During this period of rest, the body shifts into an anabolic state to restore its many systems (such as the endocrine, immune, and nervous systems) and promote the growth, repair, and regeneration of its cells and tissues.2,3 The exact mechanisms by which sleep exerts its beneficial functions are not fully understood, although its impacts on health are well documented.3 Not getting sufficient or restful sleep has been associated with an increased risk of many chronic health issues such as cardiovascular disease, type 2 diabetes, obesity, and depression.1
Sleep: an overview
It is recommended that adults get at least 7 hours of sleep every night, although the optimal duration of sleep can vary between individuals.1,4 Unfortunately, data indicates that 1 out of every 3 Canadian adults is not reaching the minimum sleep recommendation.5 Moreover, 43% of men and 55% of women experience sleep disturbances, such as issues falling or staying asleep.5 In children and adolescents, the recommended amount of sleep is higher: 9–11 hours of sleep/night for children (5–13 years old) and 8–10 hours of sleep/night for teenagers (14–17 years old).6 Sleep is also a concern among Canadian children, with around 1 out of 4 not meeting the minimum sleep guidelines.6
Certain general recommendations have been provided to help improve sleep:
- Maintaining a regular sleep schedule throughout the week;1
- Ensuring that the bedroom is calm and dark during the night;1
- Removing electronic devices, such as televisions or computers, from the bedroom;1
- Engaging in regular physical activity;1
- Taking time to relax before bedtime (and avoiding the use of screens an hour before bed);7
- Avoiding large meals, caffeine, or alcohol before bed.1
Sleep and diet
Increasing interest is being dedicated to the complex relationship between diet and sleep, with preliminary evidence suggesting that sleep not only impacts diet, but dietary behaviours may also affect sleep.8,9 Insufficient or poor-quality sleep has been associated with increased food intake and lower diet quality in both children and adults, which is believed to be due to its impacts on hormones (including hunger and satiety hormones), metabolism, and behaviour.8-12
Interestingly, emerging data is now indicating that certain dietary behaviours and choices may help improve nighttime sleep outcomes.8-12 Research relating to nutrition and sleep focuses not only on the contents of the diet but also the timing of food intake in relation to the body’s internal clock, an evolving field of study known as “chrononutrition”.
The findings of early studies suggest that certain dietary patterns may be linked to improved sleep.10 Adopting a healthy lifestyle, which includes regular physical activity and a balanced diet, is a recommended habit for healthy sleep.13 An example of a dietary pattern that has shown promise as a complementary approach supporting improved sleep is the Mediterranean diet.14,15 Currently, evidence on which dietary patterns modulate sleep is not sufficiently robust however, to inform formal sleep-specific recommendations.10
Certain foods, such as tart cherries, kiwifruit, and milk, have also been investigated for their sleep-promoting properties due to their content in certain key nutrients and compounds.10
Dairy and sleep: an overview
Dairy products, particularly milk, are often described as “sleep-promoting” foods.4,10, 16 Specifically, the advice to consume a glass of warm milk has been shared for years, and research has sought to elucidate the science behind the longstanding association between dairy and sleep.
In a 2020 systematic review of 14 studies including over 10,000 participants, researchers note that many dairy-based products have been associated with small favourable effects on sleep quality (sleep efficiency, duration, latency, and awakening).17 However, in addition to being limited in numbers, these findings were predominantly obtained using dairy products that were enriched in key compounds, such as tryptophan. Consequently, these results may not be generalizable to regular dairy products, such as milk. Currently, the evidence on the impact of regular milk on sleep is scarce and no recommendations can be emitted.
A 2014 cross-sectional study of 421 Japanese adults investigated the associations between physical activity, milk product intake, and difficulties initiating sleep.18 Results showed that participants who had a higher milk intake, particularly those who also engaged in regular physical activity, were less likely to report issues falling asleep.
While some studies have yielded promising results, evidence from large-scale or experimental studies supporting the sleep-promoting qualities of dairy remains limited.
Dairy and sleep: potential mechanisms
Milk products have several properties that are hypothesized to be involved in sleep, including a higher tryptophan content, the presence of gamma-aminobutyric acid (GABA) in fermented dairy, and several compounds that may interact with the microbiome (brain-gut-microbiome axis).17-23
A protein contained in milk, known as alpha-lactalbumin, is composed of high levels of tryptophan, an amino acid and precursor to the sleep hormone melatonin.19 In fact, alpha-lactalbumin contains the highest level of tryptophan among commonly consumed foods.19 Certain preliminary studies suggest that consuming alpha-lactalbumin before bed may help improve sleep quality.20,21 However, more research is needed to clarify this relationship and the necessary amount to yield beneficial properties, as it may exceed the level naturally contained in milk. Moreover, the brain’s absorption of tryptophan increases with blood sugar, but the amount of carbohydrates needed to optimize its absorption and improve sleep remains uncertain.7
Certain probiotic strains contained in fermented milk products, such as lactobacillus spp., produce GABA, a substance that is also generated by the brains of mammals during sleep.22 GABA acts as a chemical messenger that inhibits neuronal activity in the brain, regulates heart rate and helps promote sleep.22,23 While the role of GABA in sleep is relatively well understood, the impacts of oral GABA in humans (contained in food or in the form of supplements) are not well documented.22
Lastly, milk products are rich in a variety of unique compounds that may have beneficial properties on the microbiome through a variety of complex physiological pathways, such as milk fat globule membrane and lactoferrin.23 There is emerging evidence that these bioactive compounds may help increase resistance to stress-induced sleep issues by promoting a balanced microbiome.23 While many such compounds have yielded promising results in early studies, more high-quality studies in humans are necessary to reach further conclusions.
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5. Statistics Canada. 2017. Duration and quality of sleep among Canadians aged 18 to 79. https://www150.statcan.gc.ca. Accessed December 29, 2021.
6. Government of Canada. 2019. Are Canadian children getting enough sleep? Infographic. https://www.canada.ca.
7. National Health Service. 2019. How to get to sleep. https://www.nhs.uk. Accessed December 20, 2021.
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13. American Academy of Sleep Medicine. 2020. Healthy Sleep Habits. https://sleepeducation.org. Accessed December 20, 2021.
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20. Ong JN et al. Sleep quality and duration following evening intake of alpha-lactalbumin: a pilot study. Biological Rhythm Research 2017;48:507-517.
21. Markus CR et al. Evening intake of α-lactalbumin increases plasma tryptophan availability and improves morning alertness and brain measures of attention. The American journal of clinical nutrition 2005;81:1026-1033.
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23. Yu L et al. Beneficial effect of GABA-rich fermented milk on insomnia involving regulation of gut microbiota. Microbiological research 2020;233:126409.
24. Thompson RS et al. Dietary prebiotics and bioactive milk fractions improve NREM sleep, enhance REM sleep rebound and attenuate the stress-induced decrease in diurnal temperature and gut microbial alpha diversity. Frontiers in Behavioral Neuroscience 2017;10:240.