Our brains are far from idle during sleep, actively working to clear out waste products that accumulate throughout the day. A new study sheds light on the intricate process behind this nightly “cleaning” routine, revealing a key mechanism and raising questions about the impact of common sleep aids.
Researchers have discovered that the glymphatic system, a network of vessels responsible for removing toxins like amyloid-beta and tau proteins – both linked to dementia and Alzheimer’s disease – relies on rhythmic fluctuations in blood vessel size driven by the release of norepinephrine, a neurotransmitter involved in the body’s “fight or flight” response. The findings, published February 24, 2024, in the journal Cell, suggest that these oscillations create a pumping action that facilitates the flow of cerebrospinal fluid through the brain, effectively flushing out waste.
“The brain is the only organ surrounded by a skull, which means it has a defined volume,” explained Maiken Nedergaard, a neuroscientist and co-director of the Center for Translational Neuromedicine at the University of Rochester. “So, every time the blood volume in the brain changes, the cerebrospinal fluid has to move to compensate.”
The research team tracked blood flow, fluid movement, and chemical markers in the brains of sleeping mice. They found that the release of norepinephrine caused blood vessels to constrict, reducing blood volume and allowing cerebrospinal fluid to enter the brain tissue. As norepinephrine levels decreased, vessels relaxed and expanded, pushing the fluid – and the waste it carried – out of the brain. These rhythmic contractions and expansions occurred approximately every 50 seconds, creating a wave-like effect throughout the glymphatic system during deep sleep.
While the study was conducted on mice, researchers believe the findings likely translate to humans, given similarities in brain structure. Previous research has already shown cerebrospinal fluid circulation in the human brain. This new work, however, is the first to identify norepinephrine as a potential trigger for this process.
Interestingly, the study also revealed that the glymphatic system’s function was significantly impaired when mice were given zolpidem, a commonly used sleep medication. The drug suppressed the rhythmic oscillations, hindering the flow of cerebrospinal fluid. This discovery has prompted further investigation into the potential effects of sleep aids on the brain’s natural cleaning processes.
“Here’s important as it’s the first evidence suggesting that the glymphatic system, a neurobiological process that occurs during sleep, is actively disrupted by sedatives,” said Bryce Mander, an associate professor of psychiatry and behavioral sciences at the University of California, Irvine.
However, experts caution against drawing broad conclusions about all sleep medications. More research is needed to determine how zolpidem specifically affects brain clearance in humans, and whether other types of sleep aids have similar effects. Studying this in humans is challenging, as current technology makes it difficult to directly analyze the human brain in the same way as animal models.
The findings underscore the importance of prioritizing quality sleep for brain health. “We necessitate to rethink how we evaluate sleep medications to ensure they aren’t disrupting fundamental sleep-supportive functions,” Mander added. Different classes of drugs may have different effects on the brain that are currently unknown.
Experts emphasize that individuals should not stop taking prescribed sleep medications without consulting a doctor. However, this new research adds to a growing body of evidence suggesting caution regarding long-term use. While these medications can be a lifeline for those with debilitating insomnia and other chronic sleep disorders, their long-term impact on brain health remains a complex question.
Rebecca Robbins, an assistant professor of medicine in the division of sleep medicine and circadian rhythms at Brigham and Women’s Hospital, noted that sleep medications can provide short-term relief for millions struggling with sleep problems, and adequate sleep is vital.
Adam Spira, a sleep researcher and professor at the Johns Hopkins Bloomberg School of Public Health, added that the science of sleep aids is complex. Some studies suggest a link between regular use of sleep medications and an increased risk of dementia, though establishing a direct causal relationship is difficult. It’s unclear whether the medications themselves cause neurodegenerative changes, or whether they simply mask underlying sleep problems that contribute to cognitive decline.
Spira also pointed out that some sleep medications may even have protective effects on the brain. Further research is needed to fully understand the complex interplay between sleep aids and brain health.
experts agree that sleep medications don’t replicate the benefits of natural sleep. Cognitive behavioral therapy for insomnia (CBT-I) is often recommended as a first-line treatment for persistent insomnia, as it’s often more effective long-term and can promote better sleep quality without the potential side effects of medication.
Despite remaining questions, the new discoveries from Nedergaard and Hauglund highlight that not all sleep is created equal – and we are beginning to understand what those differences mean for brain health.
