Sleep is an important function across many species of living creatures, influencing physical health and mental clarity. Many studies assert sleep has a critical function in ensuring metabolic homeostasis, and key research suggests that this occurs through the clearance of metabolic waste from the brain.¹ Given the impacts of anesthesia on the brain and the comparison of anesthesia and natural sleep, researchers continue to consider how anesthesia affects the clearance of waste from the brain.

In the seminal 2013 study by Xiu et al., researchers demonstrated states of natural sleep or sedation by anesthesia are associated with a nearly 60% increase in interstitial space, leading to increased exchange between cerebrospinal fluid and interstitial fluid. This led to increased clearance of β-amyloid, a peptide associated with Alzheimer’s disease. The restorative function of sleep is thus suggested to be a consequence of enhanced removal of neurotoxic waste products that may accumulate in the waking brain.¹ It is believed the increased waste clearance is a result of the glymphatic system, which involves the movement of cerebrospinal fluid (CSF) through the brain parenchyma, as facilitated by astrocytic mechanisms.² 

In June 2024, several English researchers challenged the age-old belief of sleep’s capacity for neural detoxification through a study on 60 adult male C57/BL6 mice. Three groups received anesthesia injections of either dexmedetomidine, pentobarbital, or a combination of ketamine and xylazine. The researchers first determined the diffusion coefficient by injecting 4 kDa FITC-dextran (a fluorescent dye) into the caudate putamen (CPu) and then monitoring the fluorescent signal as it arrives in the frontal cortex.³ Higher diffusion coefficients typically mean molecules can spread more rapidly through the tissue, which would mean faster clearance of waste products.⁴ No significant change was observed in the diffusion coefficient of 4 kDa FITC-dextran with any of the vigilance states (awake, asleep, or under anesthesia); all vigilance states reported a diffusion coefficient value of ~25 μm² s⁻¹. ³ These results did not support the argument that sleep (and anesthesia) facilitate the clearance of potentially harmful waste products from the brain.

However, brain clearance can occur through multiple means, including enzymatic degradation and cellular uptake, transport across the blood-brain barrier (BBB), interstitial fluid (ISF) bulk flow, CSF absorption through the glymphatic system, or ventricular transport.⁵ As such, the researchers again injected a small dye into the brain parenchyma through the neocortex and used photometric methods to assess the concentration of dye left in the injected area over a period of 12 hours. Mice injected with anesthesia and sleeping mice had much higher concentration peaks (which occurred around 2-3 hours) than mice injected with saline or waking mice. The large-scale difference demonstrated that brain waste clearance was in fact significantly reduced under certain types of anesthesia, as well as during sleep. The authors also measured the EEG power spectra and found a negative correlation between peak clearance (at around 2-3 hours) and delta power (0.5-4 Hz), suggesting the deeper the sleep, the lower the brain clearance.³ These experimental data indicate brain waste clearance is reduced under anesthesia and sleep, contrary to popular belief and decades of research.

Further high-powered studies are needed to determine the verity of either situation. The authors note that almost all experiments on the subject have assessed brain clearance by introducing markers into the CSF, which then move into the brain parenchyma. Under these circumstances, entry, exit, or redistribution of the marker all occur simultaneously, greatly confounding an isolated quantification of neural clearance.³ In any case, these results urge a critical reassessment of long-standing beliefs in the field, paving the way for new avenues of inquiry. 

References

1. Xie, Lulu, et al. “Sleep Drives Metabolite Clearance from the Adult Brain.” Science, vol. 342, no. 6156, Oct. 2013, pp. 373–77.  https://doi.org/10.1126/science.1241224 
2. Benveniste, Helene, et al. “Glymphatic System Function in Relation to Anesthesia and Sleep States.” Anesthesia & Analgesia, vol. 128, no. 4, Apr. 2019, pp. 747–58. https://doi.org/10.1213/ANE.0000000000004069 
3. Miao, Andawei, et al. “Brain Clearance Is Reduced during Sleep and Anesthesia.” Nature Neuroscience, vol. 27, no. 6, June 2024, pp. 1046–50. https://doi.org/10.1038/s41593-024-01638-y 
4. Le Bihan, Denis. “Apparent Diffusion Coefficient and Beyond: What Diffusion MR Imaging Can Tell Us about Tissue Structure.” Radiology, vol. 268, no. 2, Aug. 2013, pp. 318–22. https://doi.org/10.1148/radiol.13130420 
5. Tarasoff-Conway, Jenna M., et al. “Clearance Systems in the Brain—Implications for Alzheimer Disease.” Nature Reviews. Neurology, vol. 11, no. 8, Aug. 2015, pp. 457–70. https://doi.org/10.1038/nrneurol.2015.119