Relation of serotonin-melatonin levels to baby sleep in 0-6 months and mother's postpartum depression

Early infancy is characterized by the rapid maturation of serotonergic and melatonergic pathways that can influence developing sleep-wake patterns. This thesis study investigated age-related changes in serotonin and melatonin levels in infant saliva samples and mothers' breast milk samples and evaluated their relationship with infant sleep behavior. Samples were collected from infants aged 0-6 months and their mothers. Salivary serotonin concentrations were analyzed using the LC-MS/MS method, while serotonin and melatonin concentrations in breast milk samples and salivary melatonin concentrations were analyzed using the ELISA method. Infant sleep patterns were assessed using the BISQ, and mothers' mood was assessed using the Beck Depression Inventory. Salivary serotonin levels were significantly lower in infants exclusively breastfed for 0-6 months compared to those on mixed feeding (p=0.000), and when infants were divided into subgroups according to age range and feeding status, they were significantly lower in 0-3-month-old infants compared to 3-6-month-old infants (p=0.006), in infants exclusively breastfed for 0-3 months compared to infants fed both breast milk and formula (mixed feeding) (p=0.001), in infants on mixed feeding for 0-3 months compared to those on mixed feeding for 3-6 months (p=0.009), and in infants exclusively breastfed for 3-6 months compared to those on mixed feeding (p=0.000). Salivary melatonin levels were also significantly lower in infants exclusively breastfed for 0-3 months compared to 3-6-month-old infants (p=0.009) and in infants exclusively breastfed for 0-3 months compared to those exclusively breastfed for 3-6 months (p=0.05). Breast milk melatonin levels were found to be significantly higher in mothers of 0-3 month old infants compared to mothers of 3-6 month old infants (p=0.013) and in mothers of 0-3 month old infants fed mixed diets compared to mothers of 3-6 month old infants fed mixed diets (p=0.006). Our study demonstrates that feeding pattern and age-dependent circadian maturation significantly shape the serotonin-melatonin axis in early infancy. High serotonin levels in mixed-fed infants reflect increased tryptophan intake from formula, while melatonin remains low due to developmentally limited pineal function. The age-related increase in both hormones indicates maturation of the serotonergic and melatonergic systems, and these biochemical changes are consistent with sleep patterns. Overall, the findings reveal that feeding, age, and circadian development together shape early neurochemical regulation and sleep-wake behavior.