Abstract: Serotonin, one of the most important neurotransmitters in the central nervous system, is synthesized by the amino acid, tryptophan. Given that this essential amino acid is consumed in the diet, the aim of this study was to evaluate the effect of orally administered L‐tryptophan (125 mg/kg) on circadian variations in the levels of serotonin in brain and plasma. We used male Wistar rats of 14 ± 2 weeks of age (n = 240), maintained under conditions of a 12‐hr light:dark cycle, and food and waterad libitum. Tryptophan administration was by gavage in a daily single dose at 7 p.m. for 7 days. The serotonin levels were measured by ELISA every hour at night (8 p.m. to 8 a.m.) and every 4 hr during daytime (8 a.m. to 8 p.m.). The results show that in both the tryptophan‐treated and untreated groups the highest values appeared during the beginning of the darkness with a peak at 9, 10 and 11 p.m. in controls, and at 9 p.m. in the tryptophan‐treated group. After tryptophan administration, the levels of serotonin were significantly higher in the plasma and all the brain regions analysed than in the control group. This increase of serotonin levels was greater in the pineal gland than in other brain regions, and the least in plasma. In conclusion, oral administration of tryptophan during 7 days enhances serotonin levels over a 24‐hr period, and produces an advance in the peak of serotonin in both plasma and different brain regions.

The present study evaluated whether the administration of cereals enriched with nutrients that are facilitators of sleep could help improve the sleep of infants who had sleep disorders at night time. Thirty infants aged 8-16 months with sleep disorders involving at least three nocturnal waking episodes took part in the study. They were given a night-time 'sleep facilitating cereal' product containing 225 mg tryptophan, 5.3 mg adenosine-5'-P, and 6.3 mg uridine-5'-P per 100 g of product. These cereals were given in a double-blind procedure lasting 5 weeks, with ingestion of the cereal between 18:00 and 06:00. In the control week, the children received a standard cereal (75 mg tryptophan/100 g product without nucleotides) dissolved in a standard formula milk (231.5 mg tryptophan, 2.6 mg adenosine-5'-P, 5 mg uridine-5'-P, per 100 g product). In one experimental week, the children received the night-time sleep facilitating cereal together with the standard formula milk. In another week, they received the sleep facilitating cereal together with a night milk specially formulated to attain the sleep rhythm (480 mg tryptophan, 8.8 mg uridine-5'-P, and 7.6 mg adenosine-5'-P per 100 g product). The three experimental weeks were separated by two wash-out weeks in which the milk and cereal administered was identical in composition to that of the control week. All the infants received a programmed writer actimeter which they wore continually, attached to their ankles, to record their motor activity. The recorded activity was used to calculate information about the time in bed, assumed sleep, actual sleep, sleep efficiency, sleep latency, immobility, and total activity. The infants receiving the enriched cereal during the time of darkness showed improvements in their sleep parameters, regardless of whether the milk they took at night was standard or enriched with tryptophan, adenosine-5'-P, and uridine-5'-P. In summary, the administration of enriched cereals led to an improvement in sleep, regardless of the type of infant milk used. These results support the concept of chrononutrition since they confirm that the sleep/wake rhythm can be influenced by diet.