Bifidobacterium longum B18 Modulates Pharmacologically Induced Sleep Parameters in Rats in Association with Altered Tryptophan–Melatonin Metabolism

Sleep disorders represent a growing public health concern, yet current pharmacological interventions are limited by tolerance, dependence, and rebound insomnia. This study evaluated the probiotic properties of Bifidobacterium longum B18 and its effects on pharmacologically induced sleep parameters in male Sprague–Dawley rats, focusing on tryptophan-related metabolism and gut microbiota composition. In vitro, B18 exhibited favourable probiotic characteristics, no detectable haemolytic activity, and susceptibility to all eight antimicrobials tested. In vivo, B18 significantly shortened barbital sodium-induced sleep latency and prolonged pentobarbital sodium-induced sleep duration (p < 0.05). Targeted metabolomic profiling of faeces, plasma, and hippocampus showed increased tryptophan and melatonin concentrations, whereas 5-HTP and 5-HT remained unchanged. B18 was also associated with lower Kyn/Trp ratios, although plasma Kyn concentrations were unchanged. Increased MT/5-HT ratios and elevated hippocampal AANAT levels were also observed, supporting an association with enhanced melatonin-related metabolism. Gut microbiota analysis showed higher abundances of Bifidobacterium and Lactobacillus in the B18 group than in the DZP group, together with lower Desulfovibrio abundance than in the CTR. Exploratory correlation analysis further showed positive associations of Bifidobacterium abundance with faecal tryptophan and melatonin levels and an inverse association with the Kyn/Trp ratio. Collectively, these findings indicate that B. longum B18 modifies pharmacologically induced sleep parameters in association with altered tryptophan–melatonin metabolism and gut microbiota composition, supporting further evaluation of B18 as a candidate probiotic for sleep-related functional applications.