Neuroscientists Uncover Sleep Timing Secrets
We've learned more about how sleep is controlled and why so many get too little.
Posted Nov 29, 2017
When I was a neuroscience student years ago taking a course on sleep the most exciting finding was that if you are awakened during the rapid eye movement (REM) phase of sleep you will often describe having a vivid dream. Why this occurred is still mysterious today. So is why we, like other animals, fall asleep, and why we need seven to eight hours to function well (10 to 13 hours in children three to five years old, according to the American Academy of Pediatrics).
Sleep cycles. You may have learned about other sleep phases, such as the slow-wave phase, when the brain waves that can be seen by electronic recording with the electroencephalogram (EEG) oscillates at the slow rate of about 0.5 to two waves per second, during which vivid dreams are rare. About six cycles that include periods of REM and slow waves typically occur during a good night's sleep.
But many people experience disrupted or insufficient sleep. Since the discovery of REM sleep in the 1950s, sleep medicine has expanded with sleep labs and specialists diagnosing those complaining of sleep disorders. Almost 50 million Americans are missing a refreshing night’s sleep. On a molecular level, a 2017 Nobel Prize was awarded for research on the genes that control the daily cycles of sleep and wakefulness.
Sleep medicine. What we now know about sleep and what is still unknown is described by Drs. Jerome Groopman in The New Yorker (October 23, 2017) and Meir Kryger in The Mystery of Sleep: Why a Good Night's Rest Is Vital to a Better, Healthier Life (Yale, 2017). Kryger, a professor at Yale Medical School, is a leader in the field of sleep medicine who has treated thousands of patients with sleep problems. Sleep apnea, for example, when breathing pauses many times during the night, is a widespread problem that is sometimes associated with snoring, leads to daytime sleepiness, and poses an increased risk of cardiac disorders.
Excessive daytime sleepiness can also be due to narcolepsy, when a person falls asleep suddenly during everyday activities like eating or working. Another condition can occur during awakening or going to sleep with a frightening but brief experience of not being able to move or "sleep paralysis."
Setting the internal clock. Kryger simplifies the neuroscience of sleep by describing a "wake gauge" that tells us that our body needs sleep, and a "body clock" or biological clock that tells us when to fall asleep and when to wake up.
The “wake gauge” is associated with the biochemical substance adenosine, which builds up in the brain during wakefulness, then promotes sleep, especially of the slow-wave kind. This chemical, as you might expect, is antagonized by caffeine. It's a byproduct of the chemical reactions that produce energy for muscles and other organs in the body.
The biological clock is centered deep within the brain, in a cluster of nerve cells abbreviated as the SCN, or suprachiasmatic nucleus. It stays synched to the daily cycles of the outside world by a complex mechanism triggered by light. We have learned that specialized nerve cells in the retina at the back of the eye react to light by sending signals to the biological clock,
In addition, the pea-sized pineal gland nearby secretes the hormone melatonin around evening, interacting with the biological clock. This mechanism, reminiscent of Rube Goldberg’s cartoon machines, is probably why artificial light can disrupt the natural sleep cycle, and why melatonin is often recommended as a supplement to promote sleep if needed.
Disrupted sleep. In addition to artificial light, sleep can also be disrupted by a long list that includes television, cell phones, alcohol, caffeine, sounds, and repetitive worries. Some of the remedies often recommended involve avoiding these disruptions near and during bedtime, exercise during the day, using white noise to mask other sounds, or cognitive-behavioral therapy to help control unneeded worrying.
This is not medical advice and it's important to ask your doctor or other health-care provider about any sleep problems you might have.
Restless sleepers may take comfort to know that a continuous period of sleep for eight hours a night is not universal. According to Groopman, there is evidence that in early-modern Europe and North America the standard sequence of sleep was segmented into two periods, with intervals in between when the person was awake, sometimes called “the watching,” during which people might pray, have sex, or read. In isolated tribes, living much like our early ancestors, afternoon naps are more common than in the industrialized world.
In the busy world today, there are more fascinating questions about the timing of sleep and wakefulness. For example, consider medical interns on 24-hour shifts, new parents with crying babies, and pilots on 16-hour flights. How do they cope? These are just a few questions about sleep that can be saved for another day.
Groopman, J. (2017), The secrets of sleep. The New Yorker, Oct. 23.
Kryger, M. The Mystery of Sleep: Why a Good Night's Rest Is Vital to a Better, Healthier Life (Yale, 2017).
Bjorness, T. & Greene, R. (2009). Adenosine and Sleep. Curr. Neuropharmacol., 7(3): 238-245.