Cortical lactate was monitored voltammetrically in freely moving rats equipped with polygraphic electrodes. Differential normal pulse voltammetric measurements were carried out using a lactate biosensor coated with lactate oxidase and cellulose acetate. Changes occurring in lactate level were in keeping with sleep–wake states. During slow wave sleep (SWS), the lactate level decreased significantly (−16.2%) vs. the spontaneous waking state (W) referenced to as 100%. During paradoxical sleep (PS), and still vs. W, it remained low (−9.0%) but this variation was not statistically significant. However, when this PS change was compared to the SWS variation, a significant increase in lactate level was then revealed (+8.5%). Finally, during the active waking (aW) triggered by a water puff stress, lactate level rose significantly in accordance with the animal activity (+53% compared to W). Long-term monitoring also allowed the determination of a circadian component in lactate production, the lowest and highest values being monitored during light and dark periods, respectively. The acrophasis of the circadian change occurred during the dark period, about 3 h after the light-off (+89%). It is suggested that during wakefulness astrocyte metabolism allows the transformation of the blood-borne glucose into lactate. The increase in this substrate observed during PS may fulfil the oxidative phosphorylation in order to supply the important ATP need of PS.