We all know that getting enough sleep promotes healthy brain function and optimal cognitive performance. Most sources recommend getting approximately 8 hours of sleep per night, but the amount of sleep it takes for you to feel refreshed should be individualized. Someone who is training for a marathon will generally need more sleep (over 8 hours) to facilitate recovery after grueling long-distance runs.
On the other hand, someone who doesn’t exercise and is relatively sedentary may benefit from getting under 8 hours of sleep or just enough to refresh their brain. Furthermore, other factors such as whether you take any supplements or medications, your diet, genetics, and circadian rhythm can have an impact on your sleep cycle. Although optimizing the amount of sleep you get is important, another factor to consider is sleep position.
Most people assume that your brain benefits from sleep regardless of whether you’re sleeping on your stomach, back, or on your side. New research suggests that you may want to think twice about sleeping on your back and/or stomach. It appears as though sleeping on your side may help your brain clear toxic waste (e.g. amyloid-plaques) and aid in the prevention of neurodegenerative diseases like Alzheimer’s.
Glymphatic Pathway: How the Brain Clears Waste During Sleep
When you sleep, your brain’s glymphatic pathway is responsible for clearing detrimental waste. The glympahtic pathway becomes increasingly active when the brain’s level of arousal decreases. The glymphatic system is known to clear waste from the CNS via cerebrospinal fluid (CSF)-interstitial fluid (ISF) exchange.
CSF enters the connective tissue within the brain and the glymphatic system clears interstitial fluid (ISF) from various compartments within the brain and spinal cord. During sleep, the arteries pulsate which dictate the degree of expansion or contraction of the brain’s extracellular space. When the extracellular space increases, it allows for more efficient clearance of waste via ISF because there’s a larger space for it to flow through.
As the extracellular space contracts during wakefulness, there’s less space for the waste to flow through, and since the glympahtic system is less active – less plaques are cleared. Think of the glympahtic pathway as the garbage man coming through to pick up the daily trash each time you sleep. Some of the waste cleared by the glymphatic pathway consists of a chemically sticky substance known as beta-amyloid, as well as tau proteins.
The clumping of a significant amount of beta-amyloid is known to inhibit neuronal signaling processes at the synapses. Beta-amyloid forms plaques that promote neuroinflammation and ultimately kill brain cells. Individuals with brains high in beta-amyloid tend to experience neurological impairment and usually develop neurodegenerative diseases such as Alzheimer’s.
Due to the fact that your brain clears a majority of waste during sleep, it is common sense that getting proper sleep benefits your brain in the short and long-term. To activate the glymphatic pathway, it appears as though some sleeping positions may be superior to others.
Best Sleep Position for Brain: Lateral (Sleeping on Your Side)
Research suggests that certain sleeping positions may result in greater clearance of brain waste (beta-amyloid and tau proteins) compared to others. A 2015 study published in The Journal of Neuroscience discovered that sleeping in a lateral position (on your side) may aid in the prevention of neurodegenerative diseases by enhancing activation of the glymphatic pathway. The research was headed by Hedok Lee (PhD), Helene Benveniste (MD / PhD), et al. from Stony Brook University.
The Effect of Body Posture on Brain Glymphatic Transport (2015)
Researchers noted that sleep position is largely subject to individual preference among both humans and animals. Despite these unique preferences, they suspected that certain sleeping positions may affect the brain differently. Specifically, they decided to investigate whether certain sleep positions allowed the brain to clear a greater amount of waste (via the glymphatic pathway) than others.
To determine how sleep position affected waste clearance, they analyzed the exchange efficiency of CSF-ISF in anesthetized rodents. This was accomplished with the usage of dynamic-contrast-enhanced MRI neuroimaging and kinetic modeling. The greater the efficiency of exchange, the greater the amount of neural waste (toxins) end up getting cleared from the brain.
The rodents were positioned to sleep in supine (back), prone (stomach), or lateral (side) positions. A technique called fluorescence microscopy was utilized along with radioactive tracers to measure clearance of beta-amyloid during sleep. The results from the study documented that CSF-ISF exchange (glymphatic transport) was significantly more efficient in the lateral sleep position compared to supine and prone positions.
- Lateral position: Sleeping in the lateral position resulted in the greatest amount of waste clearance compared to other psoitions.
- Supine position: In this position, a greater amount of waste was cleared compared to the prone position, but less was cleared compared to the lateral position.
- Prone position: In the prone position, the rats’ heads were nearly upright, making their posture similar to that during wakefulness. This sleeping posture resulted in slow clearance of the radioactive tracer, signifying reduction in the clearance of beta-amyloid and greater accumulation of toxins.
Evolutionary purpose of the lateral sleep position
Some resources estimate that approximately 70% of people prefer sleeping on their side (laterally). It is evident that sleeping laterally remains the most popular position compared to the prone and supine positions. Researchers speculate that sleeping on the side may have provided evolutionary benefit, ultimately clearing more waste than the other possible positions.
The increased clearance of waste during the lateral sleep position may have preserved cognitive function for a longer duration. The preservation of cognitive function over a longer-term results increased likelihood that a person would survive and pass on their genes. Compromised clearance of toxic waste (beta-amyloid and tau proteins) may have impaired cognition and impaired cognitive function.
- Source: http://www.jneurosci.org/content/35/31/11034.short
It is also known that neurodegenerative diseases like dementia are associated with sleep abnormalities. Those with neurodegenerative diseases often have difficulty falling asleep and staying asleep – which we now know may increase memory impairment due to insufficient waste clearance. With this knowledge, perhaps the link between sleeping pills and dementia is partially related to the fact that users aren’t getting enough sleep.
Lack of proper sleep may result in impaired waste clearance, resulting in accumulation of beta-amyloid and tau proteins over a long-term. Furthermore, assuming someone isn’t sleeping in the lateral position – they may experience even less waste clearance from the brain. Couple this with the drug’s effect on neurotransmission may significantly increase incidence and speed of neurodegeneration.
Why further research is warranted to understand effects of sleep position on the brain…
There are numerous reasons as to why further research is necessary to understand how sleep position affects the brain. Perhaps the most notable reason is the fact that this research hasn’t yet been conducted in humans. While the aforementioned results from the rodent study may also apply to humans, this cannot be confirmed.
- Animal variations: It should be speculated that not all animals may have evolved for increased waste clearance during sleep in the lateral position. Researchers may want to investigate whether other types of animals (e.g. dogs, cats, etc.) benefit more from sleeping in the supine, lateral, or prone positions. Certain types of animals may benefit more from the prone or supine positions compared to lateral positions.
- Clearance rates: While it was observed that more waste was cleared during the lateral sleep position compared to supine and prone positions – the exact rates should be mentioned. Researchers should attempt to specifically measure the clearance rates in humans and other animals and attempt to decipher the mechanisms most responsible for the clearance. Researchers may also want to attempt to determine whether certain individuals may clear waste more efficiently than others (possibly due to genetics).
- Drug development: Understanding how the glymphatic system clears waste, especially beta-amyloids and tau proteins may aid in the development of new pharmaceutical drugs. These drugs could be used to prevent neurodegenerative diseases like Alzheimer’s and/or possibly treat existing cases. Therefore more research should be conducted to understand the specific mechanisms of the glymphatic system during sleep and other ways to maximize waste clearance.
- Effects of other positions: Despite the fact that the supine and prone positions provided less efficient glymphatic transport than the lateral positions, it is possible that the supine and prone positions provide other unique benefits. Researchers solely investigated glymphatic efficiency rather than other neurological biomarkers.
- Long-term effects: It would be interesting to investigate the long-term effects of all three sleep positions in regards to waste clearance, cognitive function, mood, neurotransmission, and regional activation. A long-term investigation in both animals and humans should be conducted to determine whether sleep position has a significant impact on neurodevelopment and function.
- Mood and cognition: Researchers may want to investigate whether sleep position influences mood, cognition, brain waves, personality, regional activation, sleep quality, and ultimately waking consciousness. It would be especially interesting to determine whether altering sleep position could enhance cognitive function or improve mood among those who sleep in the prone or supine positions.
- Neurodegeneration: Despite the fact that the glymphatic system is better at clearing waste during sleep in the lateral position, is this increased waste clearance enough to prevent neurodegeneration? Could sleeping on your side every single night effectively prevent Alzheimer’s disease among those with increased genetic risk? The extent to which sleeping in the prone position could protect the brain from neurodegenerative pathologies is unknown, but should be investigated.
- Position specifics: There are many different subtypes of the three major sleep positions: prone, supine, and lateral. Some people sleep with different head elevations, tilts, and pressures from pillows. In addition, the position of other body parts may influence brain functioning during sleep (e.g. arms and legs). Humans who sleep prone generally still have similar head tilt to those who sleep in lateral positions. Assuming researchers believe that sleeping in a lateral position is superior to a prone position in terms of waste clearance, it may be largely due to positioning of the entire body rather than solely the head.
- Human research: As of now there is no human research that has investigated how sleep position affects waste clearance. Despite the fact that rodents and humans have a lot in common, results from the rodent study cannot automatically be generalized to human populations. Researchers are planning a human study to determine whether humans derive equal benefit to rats from sleeping in the lateral position.
- Right vs. left side (lateral position): Does sleeping on the right or left side result in greater activation of the glymphatic system? Would sleeping on the right side versus the left side result in greater activation of certain brain regions the following day and result in a different state of consciousness than the other? Would alternating nights between the right and left side help balance the clearance of waste? It is important to investigate the specifics of whether right or left side sleeping offers more substantial benefit than the other.
- Sleep time: Researchers may want to investigate whether sleep time influences the amount of waste that gets cleared. Would someone sleeping for 10 hours in the supine position experience equal waste clearance to someone sleeping 7 hours in the lateral position? It would be interesting to compare how time spent sleeping in each position affects the brain’s waste clearance.
Should you start sleeping on your side (lateral position)?
If you’re like most people, you probably already sleep in the lateral position. Assuming rats and humans derive the same glymphatic transport efficiency from various sleep positions, the lateral position may provide the greatest neuroprotective benefit compared to the supine and prone positions. It appears as though the supine (back) position provided more benefit than the prone (stomach) position.
Those that sleep in the prone position may want to [at the very least] consider transitioning to the lateral or supine positions to maximize the glymphatic waste clearance during sleep. The prone position is known to have detrimental effects on musculature, and now it is understood to be the least efficient position to clear waste from the brain. Sleeping in the lateral position is thought to be least taxing on the musculature, and may provide substantial neurological benefit via expedited waste clearance.