Cerebral Metabolic Changes During Sleep - PubMed (original) (raw)

Figure 1.. Main metabolic changes occurring in body and brain during sleep.

Adults spend 70–80% of the total sleep time in non-REM (NREM). NREM sleep is characterized by low muscle tonus, no food and water intake, and lower body temperature as compared with wakefulness, while oxygen availability remains high, all of which conditions may favor β-oxidation of fats over carbohydrates. Concordantly, the respiratory quotient, an indicator of whole body fuel utilization, is ~5% lower during NREM sleep, representing a higher level of lipid oxidation, compatible with the fasting state. A) In the NREM sleep state, the reduction in brain glucose metabolism stoichiometrically exceeds the reduction in brain oxygen consumption, although both glucose and oxygen availability remain high. Lactate levels are lower, reflecting decreased production from glycolysis as well as increased washout. Neuronal firing activity is lower, as is the tissue/extracellular level of glutamate. The antioxidant capacity, represented by glutathione level and glutathione peroxidase (GSH-per) activity, is increased during sleep. B) Biosynthetic pathways such as the pentose phosphate pathway, glycogenesis, de novo lipogenesis, cholesterogenesis and protein synthesis are active during sleep, restoring the metabolic pools that are utilized during wakefulness to support neuronal activity and signaling i.e. glycogenolysis, lipolysis and membrane remodeling. We propose that fatty acid oxidation is increased during sleep, replenishing tricarboxylic acid (TCA) cycle intermediates, and providing metabolites and ketone bodies for neurons. The fatty acid oxidation pathway could be fueled by FFAs dervied from circulation during the sleep state and/or by “toxic” lipid species released during lipolysis. NE is a known contributor to wake-related processes such as awareness, attention, and focus, and is known to stimulate metabolic processes in the brain i.e. aerobic glycolysis and glycogenolysis in astrocytes. In peripheral tissues, activation of adrenergic receptors stimulates lipolysis and NE blocks oxidation of long-chain fatty acids. It is presently unknown whether NE has a similar role in the brain. C) Blood levels of insulin, glucose and lactate iare increased by wakefulness-associated activities, such as muscle-movement and food intake, and are lower during the inactive sleep-state. Conversely, a rise in circulating fatty acid levels followed by an increase in ketone bodies are observed during the sleep state, when food intake is absent. Graphical illustrations and pathways are estimated based on studies listed in tables 1, 2, and 3.