We spend about one third of our lives asleep. This, if you think about it, is a pretty astounding percentage. Historically, this very important part of our brain’s health has been ignored until the last century. Partly, this because we had no method to really study the phenomenon. The EEG gave us the ability to observe the brain’s electrical activity during sleep, and beginning in the 1950s sleep research became a reality. Now, most major hospitals have sleep laboratories for diagnosing sleep disorders. In addition, neuroscientists have studied the mechanisms by which the brain produces sleep.
There is no single sleep center in the brain. Rather, there are nuclei along the brain stem into the base of the brain (hypothalamus) that are actively involved in producing and maintaining sleep. These centers are also associated with the activating systems that produce wakefulness. An old anatomic name for this arrangement of neurons was the reticular activating system. One of the neurotransmitters in these nuclei is serotonin, synthesized from the amino acid tryptophan. This is perhaps one reason it is common to become sleepy after eating a meal loaded with tryptophan – turkey, pumpkin seeds, cheddar and other cheeses.
Because we are not conscious during sleep we tend to think of the state as equivalent to putting a transmission in neutral, with the foot off the gas. It really is an active process. As we fall asleep the brain waves begin to slow and transition into a state termed Slow Wave Sleep. Subjects may dream during SWS, but the images are ill formed and difficult to recall if the subject is awakened. After about ninety minutes the EEG speeds up and our limb muscles become relative paralyzed, while our eyes begin moving in rapid jerks. This is termed rapid eye movement sleep. If you’ve ever watched a pet dog sleep you may see the eyes moving behind the lids. It is during this period we have our most vivid and easily recalled dreams. REM episodes last approximately 20 minutes before quieting down to SWS. As time passes, the REM periods begin to occur more frequently but last the same duration.
Why do muscles become paralyzed during REM sleep? Scientists don’t know for sure but suspect it may be an evolutionary advantage. It wouldn’t be good news to start moving around while asleep in a tree.
There are problems that can occur if the sleep centers become active during wakefulness. One of the most commonly known is narcolepsy, a sleep disorder characterized by excessive sleepiness. Two major forms happen depending upon the presence or absence of the muscle paralysis that accompanies REM sleep. If an attack of REM occurs during wakefulness, the patient can become extremely weak and even fall. This is known as cataplexy. Without the REM component, patients may just fall asleep at their desk or on an assembly line, which obviously can result in severe workplace accidents. For years this has been treated with stimulants like amphetamine. A newer drug, Nuvigil, has also been introduced.
Another cause of excessive sleepiness is inadequate regular sleep due to airway obstruction. For patients who have anatomical causes – such as overgrown tonsils – surgery may be helpful. Extreme obesity can also cause obstruction. Dickens wrote about an extremely fat person who constantly fell asleep. This is now recognized as Pickwickian syndrome.
The take-home message is that sleep disorders are now easily diagnosed and treated. But they need to be worked up at a comprehensive sleep center.