Other than getting enough sleep,

what can we do to improve the glymphatic clearance

of waste from our brains?

The provision of a running wheel,

so mice could voluntarily exercise,

has been shown to improve glymphatic clearance

in aging mice,

which was accompanied by a reduced buildup

of amyloid deposits and improved cognition.

Sleeping position may also make a difference.

Studies on rats show that their natural sleeping position,

curled up on their sides,

allows for better glymphatic transport

than sleeping on their backs or stomachs.

People also tend to spend most of their time

sleeping on their side,

particularly their right side versus left,

compared to their backs or stomachs.

This may maximize blood outflow from the brain.

When we sleep on our right side,

our right internal jugular vein,

the main blood vessel in our neck

draining blood from the head, is wide open,

and our left jugular

is partially collapsed and vice versa.

Since most people have a dominant right jugular vein,

sleeping on our right side might maximize brain drainage.

Does it matter?

Well, people with neurodegenerative disease,

mostly mild cognitive impairment and Alzheimer's disease,

tend to sleep more on their backs

than those with normal cognition.

About seventy-two percent spent at least two hours a night

on their backs compared to 37 percent of those

with healthier brains,

raising "the intriguing possibility

that head position during sleep

could influence the clearing of neurotoxic proteins

from the brain."

In crib death, sudden infant death syndrome,

sleeping position can be a life-saver,

leading to slogans like "back to sleep,"

or more morbidly, "face up to wake up."

It's premature for an adult jingle,

maybe, on your flank to not draw a blank?

The characteristic position of poor sleepers

is on their back, though,

so maybe it's the poor sleeping

rather than the position, per se,

that leads to cognitive decline,

or the causality could be reversed,

with dementia deteriorating good sleep habits.

Even if sleeping position did matter,

it may take a night in a sleep lab

to track your movements.

It turns out self-reported sleep positions are often false.

Should brain benefits to side sleeping ever be established,

you can train yourself with so-called "positional therapies"

such as the "tennis ball technique,"

which involves wearing a shirt to bed backwards,

with a ball stuffed in the chest pocket.

The uncertainties don't end with sleeping position.

The glymphatic mechanism itself

was rapidly embraced in scientific circles

and the popular press,

however, it's been controversial.

It wasn't until 2019

that the first evidence was published

that the glymphatic system discovered in rodents

even existed in human brains.

Even the relationship between sleep and Alzheimer's disease

is perhaps best summed up in a recent neurology review

entitled "It's complicated."

Yes, those getting less than 7 hours of sleep

may have higher rates of Alzheimer's,

but those getting more than 8 are at higher risk too.

If anything,

population studies show that longer sleep durations,

more than 8 or 9 hours,

are more strongly linked to Alzheimer's disease

and dementia in general

than sleeping less than 5 or 6 hours.

The association between dementia and long sleep duration

could be reverse causation,

where prodromal changes in the brain

years before Alzheimer's is diagnosed

causes prolonged sleep.

Long sleep duration may also just be a confounding factor,

a marker of some underlying health problem

that's the real culprit.

For example, oversleeping may be a sign of depression,

which itself is an established risk factor for dementia.

But there is a plausible biological mechanism

for how extended sleep duration

could increase dementia risk directly.

Longer sleep duration,

typically defined as sleeping more than 8 hours a night,

is associated with signs of systemic inflammation,

elevated levels of C-reactive protein and Interleukin 6,

and both of those inflammatory markers

in turn are associated with an increase in dementia risk.

So much more needs to be teased out

about the role of the glymphatic system

before we make conscious efforts to tweak it.