Daytime Ally, Nighttime Rival: How L-Theanine Optimizes Sleep Structure and Synergizes with Caffeine for Daytime Cognitive Function

L-Theanine is an amino acid naturally occurring in tea leaves. As a non-sedative dietary ingredient, it demonstrates unique advantages in improving sleep and daytime functioning. Unlike traditional sedatives, L-theanine’s core function lies in optimizing sleep architecture rather than merely prolonging sleep duration. Its benefits extend into the daytime, manifesting as more stable mood, enhanced attention, and improved executive function. These effects stem from its unique pharmacokinetic profile and multi-targeted central mechanisms. L-Theanine exhibits rapid absorption, with its central peak occurring later than peripheral absorption. This dual profile underpins its ability to deliver both immediate relaxation and sustained sleep enhancement. Mechanistically, it bidirectionally modulates the glutamatergic system in a state-dependent manner, endogenously enhances GABAergic inhibition, and coordinates monoaminergic homeostasis to rebalance neural excitation and inhibition. Most significantly, L-theanine exhibits a time-dependent bidirectional interaction with caffeine: synergistically enhancing daytime cognition while counteracting caffeine’s sleep disruption at night. In summary, L-theanine achieves synergistic improvement in sleep and daytime function through gentle neuromodulation, providing novel scientific evidence for dietary interventions in sleep disorders.