We know that people who get less sleep (or reduced sleep quality) are more likely to overeat. They are also more likely to favor energy-rich foods, high in fat or refined carbs.

However, there is considerably less research out there that investigates the other side of the equation: how the food choices that we make today affect the sleep we get tonight.

In a previous article, we discussed how certain macronutrients (carbs, fat) may affect sleep architecture. Now, we will look at whether there are specific foods that can affect how you sleep – specifically examining kiwifruit, tart cherries, and protein-rich foods.

 

1. Kiwifruit

Can eating kiwifruit at night help you sleep? Researchers at Taipei Medical Center tried to answer this novel question. Why did these researchers choose to test kiwifruit, of all things? Well, it probably had something to do with their funding – which they received from Zespri Kiwifruit. But we shouldn’t dismiss their results on that basis. Let’s take a look at what they did here.

The investigators recruited 24 subjects, all of whom reported some degree of sleep disturbance. Participants were directed to eat 2 kiwifruits, one hour before bed, every night. During this time, the subjects also kept personal records of their sleep via sleep diaries and standardized questionnaires. Their sleep was objectively measured using actigraphs – small wrist-worn sensors that collect data on movement and various aspects of sleep.

So what happened? After four weeks of data collection, researchers found the following:

• Subjects fell asleep faster. Sleep onset latency (how long it takes to fall asleep) decreased by 35%. Additionally, sleep efficiency – the amount of time spent asleep relative to the total time spent in bed – increased 5.41%.
• Subjects got more overall sleep. Total sleep time increased 13.4%.
• Subjects slept better. The amount of time spent awake after initial sleep onset dropped 28.9%, and their scores on the subjective sleep questionnaire declined 42.4% (in this form, lower = better sleep).

This sounds…pretty awesome. But there are two major problems with this study that should give you pause.

First of all, the trial lacks any kind of a control group. That’s a huge problem! Obviously, it’s kind of tough to come up with a placebo kiwifruit, but this limitation makes it rather hard to tell if all of these findings are meaningful.

Basically, it is possible that all of these participants were one big placebo group, experiencing placebo effects. Think about it. It’s easy to imagine that these sleep-disordered volunteers might have recorded improvements in sleep simply due to wishful thinking – or may have experienced better sleep simply because participation in the study forced them to pay much more attention to their sleep patterns. This happens a lot, which is why we value randomization and control so much in experimental research.

Here’s the second problem: The researchers attributed these findings primarily to high levels of naturally occurring serotonin in the fruit. It is true that serotonin plays multiple important roles in sleep. However, serotonin consumed in food cannot cross the blood-brain barrier. So, even if this effect is “real,” it is very unlikely that serotonin from the kiwifruit is the cause here.

To be fair, the authors also suggest that micronutrients in the kiwifruit, such as folate or flavonoids, may be responsible. However, this seems very speculative to me. The researchers did not test folate status in these subjects, or any biomarkers related to inflammation or oxidative stress, or anything else that would support such a relationship. And of course, there is no control group that we can look at as a basis for comparison. We don’t really have a good answer here, which makes it tough to say for sure whether it was the kiwifruit that produced these impressive improvements in sleep.

 

2. High protein foods (just anything high in protein, whatever you like)

Okay, so we’re not so sure about kiwifruit, and food with serotonin in general is probably a non-starter. However, the amino acid tryptophan is a precursor to serotonin (meaning that the body uses tryptophan as an essential building block for that neurotransmitter). And tryptophan from the food that we eat actually can cross the blood brain barrier.

So, what about eating foods that have tryptophan in them instead? A high protein meal should work for that, right?

Not so fast. Let’s take a look at the science.

Purified tryptophan, as an isolated amino acid, does indeed raise brain tryptophan levels, and this can increase synthesis of serotonin. However, protein-rich foods that contain tryptophan generally do not.

Why not?

Well, the transport system that shuttles tryptophan into the brain doesn’t just carry tryptophan – it also carries other chemically similar amino acids. When you eat a food that contains protein, you are consuming a bunch of different amino acids, and they all compete to occupy those transporters. So there’s not a whole lot of room left for tryptophan on the bus.

Additionally, natural sources of protein contain way less tryptophan compared to other amino acids. This means that eating more protein is actually counterproductive, because tryptophan just gets crowded out even more.

So, this idea that eating a huge amount of protein (like from turkey or milk) is going to bombard your brain with tryptophan and lull you into a serotonin-induced stupor is largely mistaken. But one thing that does appear to elevate brain tryptophan levels – at least indirectly – is insulin.

Let’s look at this small study to see how it works with humans eating real food.

MIT researchers recruited 9 healthy subjects, and randomly assigned them to consume one of the following breakfasts:

• Waffles with maple syrup, orange juice, coffee with sugar (347 kcal, approx. 5g of protein and 70g carbs)
• Turkey ham, egg beaters, American cheese, butter, and grapefruit (361 kcal, approx. 47g protein and 15g carbs)

Blood samples were taken before the breakfast, and then at regular intervals thereafter (40, 80, 120, 240 minutes). Here’s what they found:

The high-carb group experienced modest increases in blood tryptophan levels, relative to other amino acids. Meanwhile, the ham&eggs group experienced a 35% drop in relative tryptophan levels after 240 minutes. This amounted to as much as 50% difference in the plasma tryptophan ratio between the groups.

The researchers also noted that the high-carb group exhibited an 11-fold higher insulin concentration at the 240 minute mark, compared to their high protein counterparts. While unsurprising, this is important because it is very likely that insulin is mediating the shift in tryptophan levels.

Why? Insulin stimulation causes the branched-chain amino acids to be taken up by skeletal muscle, but has little effect on tryptophan, which gives tryptophan more of a numbers advantage in the transport system to the brain. This is likely why several studies have found that eating high-glycemic carbs results in higher levels of tryptophan in the brain, and increased serotonin synthesis.

 

3. Tart cherries

Finally, another food that has attracted a decent amount of attention from the scientific community is tart cherries – not just for sleep but for some other intriguing health effects. The main reason? Melatonin.

As you might already know, melatonin is a hormone that regulates the sleep-wake cycle in animals. But oddly enough, it is also found in a wide range of edible plants. Tart cherries are a particularly rich natural source. And unlike serotonin, we do know from previous research that consuming food that contains melatonin does indeed elevate circulating melatonin levels.

Okay, but does that mean that eating tart cherries can help you sleep better? Researchers at Northumbria University designed an elegant study to put this to the test.  

The researchers recruited 20 healthy young people. The participants were randomly supplemented with either:

• 30 mL (approx. 1 ounce) of Montmorency tart cherry juice concentrate
• 30 mL (approx. 1 ounce) of fruit-flavored placebo soda (containing less than 5% fruit)

They were instructed to consume two servings of the assigned product, over a period of seven days. They wore Actigraphs to monitor activity and sleep parameters during the study period, and melatonin levels were measured via urine samples.

What happened?

As expected, melatonin levels in the cherry juice group went up, while the placebo group stayed roughly the same. This hormonal difference was also reflected in the sleep measures. The subjects drinking the placebo drink experienced no significant change to their sleep, but the cherry juice concentrate group experienced less daytime napping, increased sleep efficiency, and more overall sleep time – on average about 39 minutes.

There’s some reason to believe that people with sleep disorders might experience greater benefit. Another study found that older adults with insomnia who were given tart cherry juice got a whopping 85 minutes more overall sleep time, compared to when they consumed a placebo drink. So, tart cherry juice seems to have real effects on sleep – and in realistic doses that any of us can handle.

 

Key Takeaways

• We know that changes in sleep quality and time can impact eating behavior. Whether certain foods can improve sleep quality is less certain, but there are some interesting findings in the literature.
• One study suggests that eating 2 kiwifruits may promote good sleep. However, the purported underlying mechanisms are iffy, and the study itself lacks rigor. We probably shouldn’t make too much of these findings until some more research is done. That having been said, I fail to see any potential risk in eating two kiwifruits before bed, and kiwifruit has a lot of other potential health benefits associated with it. So there’s probably no harm in trying it.
• A small body of evidence seems to suggest that a low protein + high carb meal at night could enhance tryptophan uptake in the brain, and result in greater synthesis of sleep-promoting hormones like serotonin. It’s worth remembering that overall food volume probably plays a role here as well. It is well documented that large meals in general – as most of us know all too well – can make you sleepy too, for multiple reasons. Food coma is a real thing.
• Consuming 2 ounces of tart cherry juice (or more) can increase sleep – likely due to naturally occurring melatonin and flavonoids in the fruit. If you want to try this, make sure to use tart cherries as they have many times more melatonin than the sweet ones, and Montmorency cherries seem to have more than other varieties. The juice is pretty sour so you’re going to want to dilute it with water. You can eat the fruit too, but you’d probably need to consume an awful lot to get the same dose.
• On a more general note, sleep problems may be associated with low-grade inflammation and oxidative stress. If that is the case, then it would make sense that eating fruits like kiwis and cherries would be helpful. At the very least, it won’t hurt.
• These things work reciprocally. An overall healthy diet may help you to get better sleep – and better sleep may help you to maintain a healthier diet.

 

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