Whether we hit the snooze button three times before getting out of bed for work, or we catch a quick catnap during the day, or we sleep in on the weekend to try and make up for sleep debt accumulated during the week, there’s no doubt: humans love to sleep. And for good reason. Getting good quality and quantity of sleep is good for the body and mind. It may help reduce levels of stress hormones, improve memory, and even help us resist cravings for sweets. A wealth of evidence links chronically insufficient sleep with a host of negative health outcomes, including metabolic syndrome, obesity, insulin resistance and diabetes. Getting enough sleep also helps to normalize some of the hormones involved in appetite regulation.
So the importance of sleep for overall health is well established. But what if there’s more to this equation than how much and how well we sleep? What if our sleep position also plays a role in the restorative actions of sleep? This isn’t a right-side/left-side of the bed question, or a matter of whether you sleep uncovered or with a quilt up to your neck. Rather, new research is emerging that suggests sleeping on one’s side, rather than prone or supine, may help the brain “clean house” during the night.
Researchers from Stony Brook University studied the effect of sleep position on the brain’s capacity to clear wastes, and they determined that the lateral position was most effective. The study was conducted in anesthetized rats, so any findings should be interpreted with caution and are not immediately transferable to humans. Nevertheless, the study illuminates interesting information.
The major way by which waste products—as well as glucose, amino acids, lipids, and other compounds—are transported between the cerebrospinal fluid and interstitial fluid is through the glymphatic system, somewhat akin to the lymphatic system in the periphery. The glymphatic system is most active during sleep and is largely disengaged during waking hours. In fact, researchers speculate that the biological need for sleep—which has been identified across all animal species—might be due to the brain’s requirement for this “down time” in order to eliminate potentially neurotoxic wastes.
In the Stony Brook study, rats that slept on their sides had more effective clearance of potentially harmful compounds from the brain than rats that slept supine. Rats that slept in the prone position had the least effective clearance, and the study authors speculate that this is because placement of the head during sleep in the prone position most resembles the head position while upright, during waking hours, which might fool the brain into thinking it’s still awake.
If similar findings hold true for humans, this research may have promise for treatment of neurodegenerative conditions, specifically, Alzheimer’s disease (AD). Sleeping on one’s side certainly wouldn’t be enough to prevent or reverse cognitive decline, but quantity and quality of sleep does play a role in the pathogenesis of AD, and improving sleep could be considered one small part of a multi-pronged approach to targeting the condition, particularly because there are virtually no effective pharmaceutical interventions.
Chronic sleep deprivation increases the deposition of the amyloid plaques associated with AD, and increased sleep duration may help the brain clear more plaques. This could be due to the heightened activity of the glymphatic system during sleep, and it is also likely due to the role of insulin-degrading enzyme (IDE) in clearing the plaques. The two main substrates for IDE—insulin, and β-amyloid—compete for the enzyme. However, the affinity of IDE for insulin is much greater than for β-amyloid, such that whenever there are appreciable amounts of insulin in the blood, the ability of IDE to break down β-amyloid will be inhibited. During a long period of sleep, insulin levels will come back to a relatively low baseline, or, in the case of severely insulin resistant or hyperinsulinemic patients, insulin levels may not “normalize,” but they would be lower than typically found throughout the rest of the day. At this point, when insulin levels are lower, insulin will no longer be competitively inhibiting IDE, thus allowing the enzyme to focus on degrading the amyloid proteins.
Our affinity for catching a little shuteye whenever we can might be an evolutionarily conditioned mechanism for ensuring that our brains have time to “take out the trash” on a regular basis. Overall, wastes are cleared more effectively during any kind of sleep than during wakefulness, so good sleep quantity and quality are key. The lateral sleeping position may simply impart an extra boost to this critical housekeeping function in the brain.