BACKGROUND: The purpose of the present study is to investigate effects of tryptophan intake and light exposure on melatonin secretion and sleep by modifying tryptophan ingestion at breakfast and light exposure during the daytime, and measuring sleep quality (by using actigraphy and the OSA sleep inventory) and melatonin secretion at night.
METHODS: Thirty three male University students (mean ± SD age: 22 ± 3.1 years) completed the experiments lasting 5 days and 4 nights. The subjects were randomly divided into four groups: PoorDim (n = 10), meaning a tryptophan-poor breakfast (55 mg/meal) in the morning and dim light environment (<50 lx) during the daytime; RichDim (n = 7), tryptophan-rich breakfast (476 mg/meal) and dim light environment; PoorBright (n = 9), tryptophan-poor breakfast and bright light environment (>5,000 lx); and RichBright (n = 7), tryptophan-rich breakfast and bright light.
RESULTS: Saliva melatonin concentrations on the fourth day were significantly lower than on the first day in the PoorDim group, whereas they were higher on the fourth day in the RichBright group. Creatinine-adjusted melatonin in urine showed the same direction as saliva melatonin concentrations. These results indicate that the combination of a tryptophan-rich breakfast and bright light exposure during the daytime could promote melatonin secretion at night; further, the observations that the RichBright group had higher melatonin concentrations than the RichDim group, despite no significant differences being observed between the PoorDim and RichDim groups nor the PoorBright and RichBright groups, suggest that bright light exposure in the daytime is an important contributor to raised melatonin levels in the evening.
CONCLUSIONS: This study is the first to report the quantitative effects of changed tryptophan intake at breakfast combined with daytime light exposure on melatonin secretion and sleep quality. Evening saliva melatonin secretion changed significantly and indicated that a tryptophan-rich breakfast and bright light exposure during the daytime promoted melatonin secretion at this time.
