Brain Toileting: Flushing Out the Purpose of Sleep

By Melissa S. Lim, M.D., FAASM

When new healthcare students look at me quizzically, as I recommend “pulmonary toilet measures” in the care of a patient, I am sure their look also connotes a tinge of, “What is this dinosaur of a doctor saying?” When we speak of old fashioned ‘pulmonary toileting’, we refer to the various ways in which a patient, often with the assistance of a respiratory therapist, cleans out their lungs, whether it be with increased physical activity, medications and bedside tools to break up mucus, or tipping a patient head down and beating them gently on the back. After some combination of these methods, the lungs are better cleared and cleaned of thick mucus and cellular debris. These methods have been known for decades as tools for assisting patients in the natural function of their lungs, especially when the lungs are compromised by respiratory infections.

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In the area of sleep medicine, we have yet to answer the fundamental question as to the purpose of sleep. We know that animals universally undertake this activity, despite sleep’s inherent dangers in the dog-eat-dog animal kingdom. It’s no surprise that giraffes evolved to sleep 30 minutes a day (and do not lie down for more than 5 minutes!), whereas tigers sleep for 18 hours a day. Given its universality, then, sleep must be important. Some theories include the energy conservation theory, which is supported by a decreased caloric consumption that occurs during sleep.

A second theory is that sleep serves a restorative function. For example, nocturnal growth hormone secretion may facilitate cell regeneration and muscle growth. More recently, researchers have described the metabolic clearance of toxic waste that occurs during sleep, or a form of “brain toileting,” to be concise. In animal models of sleep, there is a demonstrable increase in the volume of the fluid outside of and in between brain cells (Xie et al, Science, 2013 October; 342 (6156): 373-7), and in the clearance of beta-amyloid, one of the proteins found in abundance in brains of patients with Alzheimer’s disease. Dr. Maiken Nedergaard of the University of Rochester refers to a “glymphatic system,” much like the body’s lymphatic system, whereby the brain’s glial cells drive the exit of brain’s cellular byproducts into the cerebrospinal fluid for removal, and discovered that this system is the most active during sleep (see “Goodnight. Sleep Clean.” by M. Konnikova, NY Times, Jan. 10, 2014). Conversely, in sleep deprived animals, the clearance of brain toxins is significantly reduced. Although the corollary research in humans remains pending, the implications are many.

As a whole, we sleep fewer hours per night than we did generations before, and the sleep deprivation is steadily increasing. For instance in 2013, 40% of Gallup respondents said they slept 6 hours or less per night, whereas only 11% responded the same in 1942. Insomnia and insufficient sleep, from myriad causes, are prevalent in modern society, potentially increasing our risk of, or accelerating, neurocognitive decline.

Whether knowledge of the health risks of sleep deprivation and the health benefits of a good night’s sleep on a regular basis is enough to spur us to change our or our patients’ behavior is questionable. The inevitable alternative is that we will develop medications that enhance the glymphatic system despite sleep loss, or optimize its function during its quiescent state, i.e. while we are awake. At the very least, honing in on the purpose of sleep will help us educate our patients and provide additional ammunition to encourage lifestyle changes that enhance the opportunity to sleep. Until we learn to turn our lights off and take our brains to the loo, that may be the best we can do.

Melissa S. Lim, MD, is the Medical Director and founder of Redwood Pulmonary Medical Associates. Dr. Lim is board certified in internal medicine, pulmonary diseases, and sleep medicine. 

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