Central serotonergic system has multiple roles in animal physiology and behavior, including sleep-wake regulation. However, its role in the regulation of brain energy metabolism accompanying the sleep-wake state changes of animals has remained unclear. We demonstrated that in vivo optogenetic activation of raphe serotonergic neurons increased cortical neuronal intracellular levels of adenosine triphosphate (ATP), an indispensable cellular energy molecule. The serotonergic neuronal activation-induced increase in neuronal ATP level was suppressed by inhibiting neuronal uptake of lactate derived from astrocytes, a type of glial cells. The serotonergic neuronal activation increased cortical astrocytic Ca²⁺ and cAMP levels and extracellular lactate concentrations, suggesting the facilitation of lactate release from astrocytes. Furthermore, chemogenetic inhibition of raphe serotonergic neurons partly attenuated the increase in cortical neuronal intracellular ATP levels as arousal increased in mice. In conclusion, raphe serotonergic neuronal activation increased in cortical neuronal intracellular ATP levels, partly mediated by the facilitation of astrocyte-neuron lactate shuttle, which contributes to state-dependent regulation of neuronal intracellular energy levels.