Daily walnut consumption increases 6-sulfatoxymelatonin urinary levels and can improve sleep quality: a randomized crossover trial

We aimed to study the potential impact of daily consumption of walnuts on urinary 6-sulphatoxymelatonin (6-SMT) levels and sleep quality parameters. We conducted an open-label randomized crossover trial (NCT04799821) in 76 young adults (24.1 ± 3.4 years; 85.5% women) who either ingested 40 g of walnuts daily during dinner (intervention) or refrained from eating walnuts or any other nuts (control) for 8 weeks, with a 2-week washout period. Outcome variables included the determination of 6-SMT in urine samples collected in two consecutive periods: evening (from 20:00 to 23:00) and nighttime (from 23:00 to 07:00), the measurement of sleep quality parameters (latency, wake after sleep onset, awakenings, and efficiency) and daytime sleepiness (Epworth Sleepiness Scale). Tryptophan and melatonin contents of the walnuts used for the intervention were quantified by high-performance liquid chromatography. The 8-week walnut intervention significantly increased evening urinary 6-SMT concentrations (<i>p</i> = 0.029) and improved sleep latency (<i>p</i> = 0.001), while no differences were found between the baseline and control conditions. Likewise, the walnut intervention resulted in increased global sleep quality (<i>p</i> = 0.002) and lower daytime sleepiness (<i>p</i> = 0.002). Partial correlation analyses indicated that during the intervention, evening urinary 6-SMT concentrations were significantly associated with higher sleep efficiency (<i>p</i> = 0.026) and an improved global sleep quality (<i>p</i> = 0.006). Our findings highlight the potential of walnuts as sleep-promoting foods among young adults. Specifically, we demonstrated that a daily serving of 40 g of walnuts increases urinary 6-SMT levels, reduces sleep latency, and improves global sleep quality. Further research is needed to fully understand the underlying mechanisms involved in the diet-sleep association.