Sleep - we all love it but we typically don't get enough of it. In bustling cities such as Hong Kong, where life is fast and furious, sleep can seem like a luxury left to be savoured only on weekends and holidays.
We all know that sleep is good for us, and now scientists in the US report new evidence why: the brain changes when we sleep to clean out harmful toxins that have built up during the day.
Lulu Xie and fellow neuroscientists at the University of Rochester Medical Center used new medical imaging techniques to observe the brains of mice that were either asleep or awake. Scientists knew that the amount of energy consumed by the brain does not decrease much while we sleep, but they did not know how that energy was being used.
The brain has a unique method of waste removal, known as the glymphatic system, a kind of plumbing system that washes away toxins by pumping cerebral spinal fluid (CSF) through spaces between brain cells. Interestingly, Xie and colleagues found that the glymphatic system was almost 10 times more active during sleep.
In addition, they saw that cells in the brain "shrank" by 60 per cent during sleep. This contraction creates more space between the brain cells and allows CSF to wash more freely through the brain tissue.
By contrast, when the brain is awake, cells are closer together, restricting the flow of CSF.
CSF washes away toxic metabolites, degradation products that brain cells secrete during regular tasks in waking hours. The timely removal of waste from the brain is essential because accumulation of toxic proteins such as beta-amyloid can lead to Alzheimer's disease.
In fact, almost every neurodegenerative disease is associated with the accumulation of cellular waste products.
By tagging beta-amyloid with fluorescent tags, the scientists saw that this waste protein flowed out of the brain twice as fast during sleep. In fact, the flow of waste out of the brain during the waking state was only 5 per cent of that when the mice were asleep.
"This study shows that the brain has different functional states when asleep and when awake," says Dr Maiken Nedergaard, who supervised the study. "In fact, the restorative nature of sleep appears to be the result of the active clearance of the by-products of neural activity that accumulate during wakefulness."
Importantly, the brain can only clean itself during sleep. "The brain only has limited energy at its disposal and it appears that it must choose between two different functional states: awake and aware, or asleep and cleaning up," says Nedergaard.
"You can think of it like having a house party. You can either entertain the guests or clean up the house, but you can't really do both at the same time."
In addition to detoxification, the brain also produces brain support cells during sleep. Known as oligodendrocytes, these support cells produce myelin that wraps around neurons and allows electrical impulses to move rapidly between them, similar to how insulation works around electrical wires.
Dr Chiara Cirelli from the University of Wisconsin, Madison, reported in September that during sleep, genes promoting myelin formation were switched on and the reproduction of oligodendrocyte precursor cells (OPCs) doubled. The growth of OPCs was particularly high during the rapid eye movement (REM) stage of sleep, which is associated with dreaming.
"It is clear that the way other supporting cells in the nervous system operate changes significantly depending on whether the animal is asleep or awake," says Cirelli.
Sleeping not only consolidates memory of the day's events, it also helps improve learning, for example, of movement tasks such as playing the piano.
Using three different kinds of brain scans, researchers from Brown University in the US identified exact locations of brain activity during sleep after learning a finger-tapping task, and quantified changes among brainwaves in these regions.
The supplementary motor area (SMA), a region on the top-middle of the brain, was found to be the site of brainwave changes. Specifically, these changes were in fast-sigma and delta brainwave oscillations, which govern changes within the SMA and also changes in the SMA connectivity to other brain areas.
"It's an intensive activity for the brain to consolidate learning and so the brain may benefit from sleep, perhaps because more energy is available or because distractions and new inputs are fewer," said Dr Yuka Sasaki, who led the study.
But it turns out too much sleep is also harmful. A new study by the Centers for Disease Control Prevention (CDC) in the US links too little sleep (six hours or less) and too much sleep (10 hours or more) with chronic diseases in adults aged 45 years and older.
Published in early October this year, the study involved more than 54,000 participants in 14 states in the US. Both short and long sleepers reported a higher prevalence of coronary heart disease, stroke, diabetes, obesity and anxiety, compared to optimal sleepers who got seven to nine hours of shut-eye on average.
In fact, for long sleepers, the association with coronary heart disease, stroke and diabetes was even stronger with more sleep.
"Sleeping longer doesn't mean that you're sleeping well. It is important to understand that both the quality and quantity of sleep impact your health," says Dr Safwan Badr, president of the American Academy of Sleep Medicine (AASM).
"It's critical that adults aim for seven to nine hours of sleep each night to receive the health benefits of sleep, but this is especially true for those battling a chronic condition," he adds.
Getting the optimal amount of sleep is not only good for our health, but also apparently for our looks. Sleep apnea is a condition where snoring and breathing interruptions disrupt sleep, preventing patients from getting a good night's rest.
Affecting millions of adults, most of whom remain undiagnosed, sleep apnea increases the risk of heart-related problems and daytime accidents. A common treatment for sleep apnea is known as continuous positive airway pressure (CPAP) treatment, which can stop snoring, reduce blood pressure and improve daytime alertness.
Patients who have undergone CPAP treatments for several months were found to also have specific improvement in facial appearance. In other words, sleeping better made these patients look better.
Doctors at the University of Michigan used a variety of ways to assess appearance. Firstly, they used a precise face-measuring system called photogrammetry to assess tiny differences in facial contours before and after CPAP treatment.
They also mapped the colours of patients' facial skin. Finally, a subjective test of appearance was used in which 22 independent raters were asked to rank the attractiveness, alertness and youthfulness of patients by looking at photos, without knowing which were "before" and which were "after".
The overall result of the study was that patients appeared more alert, more youthful and more attractive after sleep treatment, and objective measures of facial appearance showed less redness and puffiness.
And so scientific research returns the verdict that the right amount of sleep is good for our brains and improves our looks.
The next time you're faced with the dilemma of going to bed or squeezing in a little more work, you now know the reasons why you should choose sleep. Sweet dreams.