Tossing and turning? A handful of nuts could help you get some much-needed z’s.
That’s according to a new study published in Food and Function, which found that eating 40 grams of walnuts (about 1/3 cup) with dinner for eight weeks improved melatonin production, sleep quality, and daytime alertness in young adults.
While nuts might not be the first thing that comes to mind for better sleep, previous research has also linked walnuts with improved rest. But according to study author Maria Izquierdo Pulido, this new paper goes a step further.
“Our study is the first to provide experimental evidence through a nutritional intervention, demonstrating a causal relationship between daily walnut consumption and improved sleep quality,” Izquierdo Pulido, also a professor of food science and gastronomy at the University of Barcelona, told Health.
To examine the link between walnuts and sleep, Izuierdo Polido and her colleagues recruited 76 young adults aged 20 to 35 to participate in an 18-week trial.
In the first phase of the study, which lasted eight weeks, participants were divided into two groups: one that consumed 40 grams of walnuts with dinner each day, and one that did not. (During this time, participants were told not to eat other types of nuts.)
After a two-week washout period, the groups switched: Those who had been eating walnuts stopped, and the other group began eating them.
Throughout the study, participants provided urine samples to measure melatonin production. For seven consecutive days in each phase, they also wore wrist trackers that monitored their sleep patterns, activity levels, skin temperature, and light exposure. Finally, they were assessed using a global sleep quality score, which measured four aspects of nighttime rest:
- Sleep latency (the time it takes to fall asleep)
- Sleep efficiency (the time spent asleep as a percentage of the time in bed)
- Number of awakenings
- Amount of time spent awake after falling asleep
During the walnut-eating phase, participants experienced significant increases in evening melatonin production, took less time to fall asleep, and scored higher on global sleep quality. They also reported less daytime sleepiness.
The study did have some limitations, though. It was relatively small, and funded by the California Walnut Commission (though the authors said the organization had no role in the study’s design or data interpretation).
Because participants knew whether or not they were eating walnuts, their beliefs or behaviors may have been influenced. Additionally, all participants were instructed to eat a Mediterranean-style diet during the trial, so other foods could also have played a role in the sleep improvements.
Again, the study has some caveats. But if eating walnuts really does improve sleep, as the research suggests, it may be due to the nuts’ hormone content.
For the study, the research team also analyzed the walnuts’ levels of sleep-friendly compounds, and found some impressive results. Per serving, walnuts contained an average of 84.6 milligrams of tryptophan, 118 nanograms of melatonin, and a tryptophan-to-competing amino acid (CAA) ratio of 0.058.
Each of these factors is meaningful for better rest, behavioral sleep medicine psychologist Daniella Marchetti, PhD, DBSM, told Health. Tryptophan is “the biochemical starting point” for the synthesis of serotonin and melatonin, both of which regulate sleep, she explained.
As for melatonin itself, it’s known as the “sleep hormone” for a reason. “Melatonin rises in the evening to signal that it is nighttime, which helps you fall asleep. [Therefore], eating foods with melatonin can boost these signals to help the body prepare for sleep,” Marchetti said.
The ratio of tryptophan to CAA also plays an important role. “If you have a lot of amino acids competing for space to get through the ‘doorway’ in the brain, tryptophan usually loses out to its competitors,” Marchetti said. “Having a high CAA ratio means that tryptophan has a better chance at crossing the blood-brain barrier.”
There are plenty of good reasons to snack on walnuts—like supporting heart health and cognition, registered dietitian Lisa Valente, MS, RD, told Health.
Getting more sleep might be another rationale for chowing down them. But Marchetti cautions against putting too much stock in a single snack. “These results are strong enough to indicate that walnuts are a sleep-promoting food, but they would not be considered an adequate substitute for insomnia treatment at this time,” she said.
If you do opt for walnuts in the evening, she recommends 30 to 40 grams (about a small handful) after dinner or as a bedtime snack. This timing gives your body time to experience a rise in melatonin, which can take two to three hours after consumption.
