Reviewed by Dr. Michael Leon, PhD, Professor Emeritus of Neurobiology and Behavior, University of California, Irvine.
Most of us know the slow and foggy feeling after a bad night of sleep. Part of the reason may be that your brain never finished its nightly cleanup.
Every cell in your body produces waste as it works. In most of the body, a network of vessels called the lymphatic system collects that waste and carries it away. For a while, scientists believed the brain didn't have one.
It turns out that it does. It works mostly while you sleep, and one of its drainage routes runs along the nerves in your nose.
What is the glymphatic system?
In 2012, Maiken Nedergaard's lab at the University of Rochester described a waste-clearance network in the brain and named it the glymphatic system (after the glial cells that are involved with it and the lymphatic system it resembles).
Here is how it works. Cerebrospinal fluid, the clear fluid that surrounds your brain and spinal cord, flows into the brain along the outside of its arteries. It washes through the spaces between brain cells, picks up waste, and carries it back out.
One of the things it removes is beta-amyloid, the protein that clumps into plaques in Alzheimer's disease. When the researchers studied mice missing a water channel the system depends on, those mice cleared beta-amyloid far more slowly.
Your brain does its cleaning while you sleep
The glymphatic system is one of the reasons why nighttime is the best for olfactory stimulation.
It was found that in sleeping mice, the spaces between brain cells widen and waste clears far faster than when they are awake.
The same thing appears to happen in people. At Boston University, 13 young adults slept inside an MRI scanner. During deep, non-REM sleep, their brain activity slowed into large, rolling waves, and right behind them, pulses of cerebrospinal fluid washed into the brain about every 20 seconds.
And this is part of the reason why deep sleep is so vital for brain health and memory. When 20 healthy adults stayed awake for one night, brain scans the next morning showed beta-amyloid had risen about 5% in the hippocampus, the brain's memory center.
Why it isn't working well for many people
The glymphatic system depends on deep sleep, and many adults aren't getting enough of it. Deep sleep declines steadily from young adulthood onward, so the cleaning window gets smaller every decade.
The drainage routes themselves also wear down. In mice, a key network of drainage vessels behind the nose shrinks and works less efficiently with age.
In addition, sleep aids can affect the glymphatic system. In mice, zolpidem, a common prescription sleep aid, reduced the flow of fluid through the brain by more than 30%, even though the mice were still asleep.
One of the brain's exits runs through your olfactory system
Once CSF has collected waste, it has to leave the skull.
At the roof of your nasal cavity sits the cribriform plate, a thin, sieve-like bone full of tiny holes. Your olfactory nerves pass through those holes, connecting the neurons in your nose to the brain. In mice, researchers at Penn State showed that cerebrospinal fluid drains out along those same nerves, through the plate, and into the tissue of the nose. Then they destroyed the smell neurons. With those neurons gone, the amount of cerebrospinal fluid flowing out through the cribriform plate dropped significantly.
Most of this work has been done in mice, and researchers are still measuring how much human cerebrospinal fluid leaves this way. But it’s enough evidence to show that the olfactory system is not only about smelling things; it can also affect how brain waste gets out.
Smell and drainage may decline together
Your smell neurons are the only neurons in your body directly exposed to the outside world, and pollution, infection, and age wear them down. Keeping them active appears to help them survive: in mice, regular odor stimulation helped olfactory neurons live longer.
Researchers are now looking at whether smell loss and poor brain drainage show up in the same people. In early findings presented in 2025, among cognitively healthy adults with a family history of dementia, those with a weaker sense of smell also showed lower glymphatic function on MRI images. That fits a pattern we already know well: smell loss tends to show up years before memory loss in Alzheimer's disease.
This is why Dr. Leon developed Memory Air. He knows olfactory enrichment supports the glymphatic system’s function. Odors delivered during sleep have been shown to deepen slow-wave sleep without waking people up, and deep sleep is when the glymphatic system activates.
The bottom line
Your brain has its own drainage system; it runs mostly during deep sleep, and one of its exits follows the nerves that give you your sense of smell. When deep sleep thins out, those nerves wear down, and the system appears to work less effectively.
Memory Air was designed around the hours when the brain does this work. It delivers a rotating sequence of 40 distinct scents while you sleep, through the one sense that reaches the brain without waking you up, during the same deep sleep that drives the brain's cleaning.