Some swear by a night on their side, others on their back. But which sleeping position is the best for a good night's rest?
If you live anywhere affected by the recent heatwaves, you may well have spent your nights tossing and turning, trying out different sleeping positions in an attempt to get comfortable. But what does the evidence say about which sleeping positions are actually the best? Lol
Studies on everyone from seafarers on container ships to welders in Nigeria might be able to help us, although given how important sleep is to us it's surprising how few large-scale studies have been conducted.
First you need a way of working out which position people are sleeping in. You can ask them of course, but we only really remember the way we were lying when we were trying to fall asleep and the position we wake up in. To find out more, researchers have tried a variety of techniques including filming people while they sleep or getting them to use wearable technology that monitors their movements.
In Hong Kong researchers are developing what they call the "Blanket Accommodative Sleep Posture Classification System", which uses infrared depth cameras that can detect a person's sleep position even through a thick blanket.
Researchers in Denmark used small motion-sensor detectors attached to volunteers' thighs, upper backs and upper arms before they went to sleep to establish their favoured sleeping position. They found that during their time in bed, people spent just over half their time on their sides, around 38% on their backs and 7% on their fronts. The older the people were, the more time they spent on their sides.
This bias towards sleeping on our sides is something we develop only as we become adults, because children over the age of three spend on average an equal amount of time sleeping on their sides, back and fronts.
Babies, meanwhile, sleep mainly on their backs because they're put in their cots this way for safety reasons.
So sleeping on your side is the most common position and we could trust the wisdom of the crowd to choose the position where they sleep best, but what about the data? A very small observational study in which people could sleep however they preferred found that those who slept on their right side slept slightly better than those on their left, followed by those on their backs.
If you find it easy to sleep on your side, then it's probably also best for anyone else trying to get to sleep nearby. On one occasion, while touring a submarine for a radio programme I was making, the submariners showed me their sleeping quarters, where the bunks were stacked so closely on top of each other that it was hard to turn over. That meant they tended to sleep on their backs, so they told me it was a race to get to sleep first before the whole cabin was full of snoring men.
Another small study looked at seafarers working on merchant container ships and found that respiratory disturbances such as snoring were more common when the seafarers were sleeping on their backs.
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Many people find it easier to drop off to sleep on their back - but it doesn't necessarily mean it's the best position to sleep in (Credit: Getty Images)
Some snoring is caused by severe obstructive sleep apnoea, where breathing stops and starts while the person is sleeping. This has been found to be more common in people who consistently sleep on their backs.
By contrast, lying on your side helps to clear the upper airway and prevents the uvula (the fleshy bit that hangs down in the back of your throat) and the tongue from obstructing the throat, leading to less snoring. Indeed, in some cases, a move from sleeping mainly on your back to mainly sleeping on your side has been shown to solve the problem of sleep apnoea altogether.
Sleeping on your side may well have other benefits too. For instance, research into the sleep patterns of welders on container ships in Nigeria showed that those who slept on their back were more likely to suffer back pain, compared with those who slept on their sides.
But this doesn't mean sleeping on your side works for everyone or is a panacea for all aches and pains. It depends on your ailment and the exact position you adopt during sleep. Researchers in Western Australia monitored volunteers' bedrooms for 12 hours a night using automatic cameras and found that those who said they regularly woke up with a stiff neck spent more time sleeping in what the researchers refer to as "provocative side sleeping positions".
This phrase might conjure up all sorts of possibilities in your mind, but what it means in this context is sleeping on your side in a twisted position, for example, with your top thigh reaching across the other thigh, twisting the spine. By contrast, people who slept in a straighter, more supported side position reported having less neck pain.
What the design of this study couldn't ascertain of course is whether sleeping in the "provocative" position was causing the neck pain or whether people were adopting this position because it was the only way they could get comfortable because of neck pain.
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Some people – like those serving on submarines – don't even have the option to sleep on their side (Credit: Getty Images)
So how about if you got people to trial a new sleeping position and then followed them to see if it made a difference to the pains they experienced?
In a study conducted with older people taking part in a fitness programme in Portugal, people with back pain were instructed to sleep on their sides and those with neck pain to try sleeping on their backs. Four weeks later 90% of the participants said their respective pains had reduced.
This looks like an impressive result, but there is a caveat. Only 20 people took part in the study – a small sample – so it's not possible to conclude that this simple change of sleeping position would have such a positive effect on everyone who suffers from back or neck pain. As ever in scientific research, more studies are needed.
If you do suffer with heartburn, it might be worth trying to sleep more on your left side in future
For one medical issue, it's not just a question of whether to lie on your back or side, but rather, which side you lie on. In acid reflux, gastric juices come up from the stomach, causing an intense burning in the chest. Sometimes doctors advise people to try sleeping on propped-up pillows in an attempt to relieve this very unpleasant kind of pain.
If the discomfort happens repeatedly it is known as gastro-oesophageal reflux disease, which can have serious consequences. Why this should have happened is not entirely clear, but one possible explanation is that sleeping on the left keeps the junction between the stomach and oesophagus above the level of the gastric acid. Sleeping on the right relaxes the lower oesophageal sphincter, allowing the acid to escape.
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Sleeping on one side is the most common psoition - and it also means you're less likely to snore (Credit: Getty Images)
Whatever the answer, if you do suffer with heartburn, it might be worth trying to sleep more on your left side in future.
So far I've concentrated on sleeping on your side or your back because that is what the majority of people do. But what about those people – a small minority – who sleep on their fronts?
Well, to start with, one study suggests it’s not a good idea if you suffer with jaw pain, which is perhaps not very surprising.
And what about wrinkles? Surely, lying with your face squashed into your pillow makes wrinkles worse?
Writing in Aesthetic Surgery Journal, a group of plastic surgeons suggest, rather poetically, that the skin on your face is best preserved if it is treated like "seaweed that sways while tethered to a stalk".
So the idea is to impose the least strain possible on your face while you sleep and that rules out sleeping face down. And if preserving your skin is most important to you than sleeping better or dealing with aches and pains or reflux, then sleeping on your side isn't ideal either.
What then can we conclude from all this? First that, all other things being equal, sleeping on your side seems to have several advantages, but your precise posture can have an effect on neck and back pain – and the side you sleep on can increase or reduce acid reflux. Snoring increases if you sleep on your back, but we all vary so it could still be the way you sleep best.
Kefir, kimchi, sauerkraut and kombucha are very different foods, but they all have one crucial thing in common: they're fermented.
Humans have used fermentation throughout history to preserve food. "Every culture has their own fermented foods," says Gabriel Vinderola, associate professor of microbiology at the National University of Litora in Argentina. "Now, fermentation is spreading. There are thousands of different types, and they're produced in a more industrial way."
Producing fermented foods on an industrial scale – rather than just in our kitchens – has its benefits and downsides. Despite fermentation eliminating the need for chemical preservatives, researchers at Kings College London recently found additives in almost one third of the fermented food products they tested from UK supermarkets.
These additives – including salt, sugar and artificial sweeteners – are within legal guidelines. But it does mean that some are technically classified as ultra-processed.
So are fermented foods actually good for our health, or just another ultra-processed food we should be avoiding?
Fermented foods are produced through controlled microbial growth, or fermentation, the breakdown of carbohydrates such as starch and sugar by using bacteria and yeast.
The fermentation process has many variables, such as the bacteria used, and the environmental conditions, which means there are thousands of different types of fermented foods. Some of the most well-known include kimchi, kombucha, sauerkraut, tempeh and yoghurt.
"The process of fermentation can produce new bioactive compounds, such as organic acids and different peptides that have different effects on our health," says Paul Cotter, senior principal research officer at Teagasc Food Research Centre in Ireland, the country’s national agriculture and food body.
Fermented food can be split into two groups: those containing live bacteria, and those with bacteria that died during production, such as some breads, beer and wine.
During fermentation, microbes usually feed from the sugar in a food, and this sugar fuels all of their biochemical reactions, Vinderola says.
"Then it will start releasing things – such as lactic acid, which is anti-inflammatory – that weren't present in the food before. It can also cut amino acid chains to release small fractions that can benefit our guts."
Even if the fermented food doesn't have any live bacteria, it's still associated with some health benefits, Vinderola says. Before they die, the microbes produce health-promoting molecules, such as peptides, he says.
The fermentation process can also reduce or remove gluten from some foods, too, which is beneficial to those with celiac disease, another gut problem.
Can fermented foods boost the immune system?
In recent decades, there have been mounting concerns among scientists that modern lifestyles may affect our immune system by altering the diversity of our microbes. "Our diets are generally low in fibre and we have a lot of antibiotics and stress, and we don't sleep well. And these factors will all impoverish the microbes in our bodies," says Vinderola.
In principle, fermented foods could change that. "The main role of fermented foods is that they can give you live microbes. These microbes get into the gut, and train your immune cells how to control inflammation," says Vinderola.
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Many fermented foods such as kimchi contain live probiotic bacteria (Credit: Getty Images)
Low-grade inflammation is an issue because inflammatory compounds can spread through the body via the bloodstream, reaching the brain, heart or liver, for example. This can cause chronic conditions, Vinderola says.
Consuming more microbes may train the immune system to better differentiate between good and bad bugs, says Cotter. When our immune systems struggle to do this, it can increase the risk of developing auto-immune diseases, he adds, such as inflammatory bowel disease.
In one recent study, researchers found eating sauerkraut – finely cut, fermented raw cabbage – could potentially have a substantial anti-inflammatory impact.
How so? Claudia Stäubert at the University of Leipzig in Germany and colleagues found that sauerkraut increases the concentration of lactic acid bacteria-derived metabolites in the bloodstream. This can activate a receptor called HCA3, which informs the immune system that foreign substances are in the body.
"This means the immune system is less activated, which is good," she says. "A bad immune system overreacts, which can cause autoimmune diseases, so it's good to eat fermented foods to train the immune system to be less responsive," she says.
Can fermented foods help with anxiety and depression?
It's possible fermented food might benefit mental wellbeing, although the research here is tentative.
The researchers analysed and compared their microbiomes and other nutrients in their guts and found that those who ate fermented foods had more bacterial diversity and higher amounts of short chain fatty acids, which are produced by bacteria, than those who didn't.
In another small study with the same participants, Gomez and colleagues found that self-reported mental health scores were more consistent across the regular consumers of fermented food, while the non-consumers had more fluctuating mood reports. These results have yet to published, however.
Gomez has a study – not yet published – comparing the effects of organic versus conventional fermented foods in the gut. He says he found a link between fermented food consumption and the neurotransmitter gamma-aminobutyric acid, especially with organic foods.
"This is an inhibitory neurotransmitter that makes you feel calm, and could be a remedy against anxiety and depression," he says.
In another yet-to-be-published study, Gomez fed mice a Western diet high in sugar and fat, and then carried out several lab tests to confirm that the mice had developed depression. He then fed half of them kombucha, and found that the symptoms improved, possibly via changes in the microbiome, compared to the mice who didn't eat kombucha.
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Kefir is made by fermenting milk and kefir grains – colonies of yeast and bacteria (Credit: Getty Images)
What about fermented foods and the risk of obesity?
Gomez has found in his research that fermented foods can produce metabolites known to help treat obesity. While this has been more widely studied, it's not clear yet what mechanisms are behind this effect. However, one explanation is that some of the nutrients in fermented food can contain metabolites that help to regulate our appetites via appetite-related neurotransmitters in the body.
Like many areas of health, researchers are now asking how fermented foods can be personalised to help people with individual health concerns.
We and other labs are taking a deep dive on specific fermented foods to learn more about how we can make them even better at enhancing health benefits," says Cotter.
"The challenge here is that someone making fermented products at home won't know which version they have, and it might not be the right version for their particular needs," he says. "There's an opportunity for more research to go down the personalised fermented route so you can harness the right microbes for your specific needs."
And while the Kings College London analysis of fermented foods in UK supermarkets found inconsistencies across the nutritional content of different brands of fermented foods, researchers are hoping their work can help improve the content of commercially available fermented foods.
For example, in the future, a better understanding of which microbes are present in different versions of fermented foods could help producers of fermented foods retain these bacteria when scaling up production.
"This has been a problem in the past," Cotter says. "People make fermented foods at home via a natural process and there are typically lots of microorganisms present. When produced on a larger scale, there's usually simplification, and they only use a few microorganisms for quality control reasons, but they may lose some health benefits along the way."
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Sauerkraut contains compounds that have an anti-inflammatory affect (Credit: Getty Images)
Are there any downsides to eating fermented foods?
Some mass-produced fermented products, such as ready-made kombucha soft drinks and teas, can also be high in sugar. And although the probiotic bacteria in fermented foods can prevent the growth of harmful microbes, there is still a risk of food poisoning bacteria in unpasteurised foods. Contaminated kimchi, for example, was associated with two large Escherichia coli outbreaks in South Korea during 2013 and 2014.
Which fermented food should you be eating?
There's very little research looking at which specific fermented foods might be the healthiest. This is because each fermented food has a different bacterial profile, depending on exactly how it was made.
"Probiotics and prebiotics are specific microbes that can be studied in clinical trials, but we don't know which microbes are in any one specific fermented food," Vinderola says. "Fermented foods have a complex community of microbes that may change from one kombucha to another."
I'd suggest buying 10 fermented foods and gradually introducing them into your diet to see which your body agrees with – Paul Cotter
The most researched fermented food is yoghurt, Vinderola says, which is always made up of two specific types of bacteria, no matter where it's made in the world (Lactobacillus bulgaricus and Streptococcus thermophilus). This makes it easy to build on previous research to form a reliable evidence base.
However, with kefir, for example, you'll get different results in different parts of the world because it will contain different bacteria, so it's difficult to compare results and build an evidence base," Vinderola says.
Given these gaps in our knowledge, should we be eating more fermented foods? Yes, says Cotter – but he advises introducing them gradually into your diet.
"I'd suggest buying 10 fermented foods and gradually introducing them into your diet to see which your body agrees with," he says. "And take a note of what you've eaten and how you feel afterwards."
This is because it can sometimes take a few days for our guts to become accustomed to certain fermented foods, and, in rare cases, it can cause a minor allergic reaction.
How often should we eat fermented food?
Gomez has found that those who've been eating fermented foods across their whole lifetime may have a permanent healthy advantage in their gut microbiome. He noted that, among the participants in his study on fermented food and mental health, one was from Korea, while the others were from the US – and this participant had gut bacteria associated with kimchi.
"The US participants probably started consuming fermented foods later in life, whereas Koreans eat a lot of kimchi, and the Korean participant was probably eating this since he was a child," Gomez says.
These findings led Gomez to wonder if there are permanent effects of consuming fermented foods over a long period of time.
"But this doesn't mean you can't still enjoy the benefits of fermented foods," he says of those who only start eating them later in life.
Whichever fermented foods you want to try, Vinderola advises eating it regularly. "Getting any health benefits depends on how frequently you eat it," he says. "You should eat it regularly, because the immune system needs constant stimulation."
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