A Tour of Sleep
Considering that people on average will spend 25 years of their lives asleep, it's surprising how little most of us know about what goes on when the lights go off. It's almost as if our waking selves and sleeping selves are two separate beings living in alternate dimensions, never catching more than a passing glimpse of each other.
Most people don't seem very curious or concerned about this odd desire they have to close their eyes, lie down, and blank out for several hours every night...until something goes wrong, or even very, very wrong. Maybe it's that they can't fall asleep, or can't wake up, or find chocolate syrup and raw noodles in their hair in the morning, or have just pummeled their spouse with jackhammer legs, and wonder what in the world happened when they thought they were in bed and supposedly asleep.
Poets and philosophers have painted contradictory images of sleep, but its resemblance to death is often a theme. Edgar Allan Poe called sleep "those little slices of death," John Fletcher named sleep "Brother to Death," and Roman poet Ovid wrote, "What else is sleep but chill death's likeness?" But maybe it's William Shakespeare, who often wrote about insomnia, who captured it best when he called sleep "nature's soft nurse."
While it may look like nothing much is happening while a person is sleeping, there's actually a complicated chain of events going on in the brain, and that chain is vital to our overall health. With all the possibilities for the complicated chain of events to break down or even go haywire, it's surprising that sleep clinics aren't exploding with more people reporting all sorts of sleep problems.
We know that at least 10 percent of the U.S. population has a clinically significant sleep disorder, but it's hard for us to know just how many people have parasomnias—because it's still such a new field of study. Most medical schools continue to allocate little time in their curricula for teaching about sleep and its disorders, so if someone goes to a family doctor with a sleep-related complaint that's out of the ordinary, it's unlikely to be diagnosed and treated properly. And worse, plenty of people don't realize that they have "real" disorders; they assume that their strange sleep behaviors are just their own weird quirks that they have to live with. Many of my own patients had no idea anyone else had ever gone through what they'd gone through until they saw a magazine article or television report where a patient from our clinic described the same problems.
There's also reluctance among many to talk about their concerns because of the potential stigma involved. Until recently, even the medical community believed that people who exhibited violent or aggressive or sexual behavior in their sleep probably had unaddressed psychological problems. Surely that man who throws punches and shouts obscenities in his sleep really has some pent-up rage he's not addressing during the day, right? As it turns out, probably not. There are rare cases of parasomnias that are caused by purely psychological issues, but we've now discovered that the vast majority of those with extreme sleep disorders have psychological profiles that look no different from the rest of the population. Their brain chemistry and behavior have gone amok during sleep.
To understand what makes things go so wrong during sleep, though, first we need to know what normal sleep looks like. So let's take a tour.
The Stages of Sleep
There are two basic types of sleep: non-rapid eye movement (NREM) and rapid eye movement (REM). NREM sleep has four stages, and REM sleep is one stage, so together, they comprise five important stages of sleep that cycle throughout the night.
Stage 1 (NREM)
This is the hazy stage when you're just falling asleep. It's a very light sleep, and It's easily disrupted—if someone turns on a light or makes a noise, you'll probably awaken. Often, if you're awakened from your first stage 1 sleep, you don't even know you were asleep. Or you may remember the experience as an interruption or a disjointed train of thought or image. Your alpha rhythm (the predominant rhythm—8 to 12 cycles per second—seen with eyes closed during relaxed wakefulness) is reduced to less than 50 percent of total brain-wave activity, your muscle tone becomes relaxed, your eyes slowly roll back and forth, and you may experience sudden muscle contractions (hypnic jerks) that jolt you awake, sometimes with the sensation that you were falling. We aren't sure yet why these contractions happen, but it's nothing to worry about. Some people, however, move rapidly or even race through stage 1 sleep and virtually jump into stages 2, 3, and 4 sleep.
Your brain activity, though mixed, is dominated by theta waves, which run at 4 to 7 cycles per second. This stage usually lasts less than 10 minutes, but can last 30 minutes or even two hours in people with sleep-onset difficulties.
To successfully enter sleep, a person must be able to disengage from wakefulness while being able to engage in sleep, a necessary two-step, mind-brain-body process with inherent vulnerabilities.
Stage 2 (NREM)
This stage is generally considered the "baseline" of sleep—the clear-cut starting point. It's difficult to pinpoint to exact second someone falls asleep because it doesn't happen all at once, and a person may report being awake when the tests appear to show stage 1 sleep, or alternating between wakefulness and stage 1 sleep.
In stage 2, you're still in a moderately light sleep, and still fairly easy to arouse, but your heart rate slows down and your temperature drops in preparation for the deeper sleep to come. Your brain activity, which can be measured by scalp electrodes in a sleep lab, will show slower waves mixed with sporadic bursts of faster waves called sleep spindlesand also a brain wave pattern known as K-complexes that look like the letter "K". Sleep spindles and K-complexes are hallmarks of stage 2 sleep. As an adult, you'll spend about half of your total sleep time in stage 2.
Stage 3 (NREM)
Now you transition down to slow-wave sleep or delta sleep. Between 20 and 50 percent of this stage is made up of high-voltage, slow-wave activity known as delta waves, which run at 1 to 3 cycles per second. Stages 3 and 4 are often grouped together because there's not much that distinguishes them from a physiological standpoint.
Stage 4 (NREM)
It's a common misconception that REM sleep is the deepest sleep. Actually, stage 4 NREM is the deepest sleep, closely followed by stage 3. When you've been sleep deprived, your body craves delta sleep and will try to make up for lost time by getting you to stage 4 faster and keeping you there longer. When people say that someone is "out like a light" or in a "dead sleep," the person is probably in stage 3 or 4, when it's very difficult to arouse someone. This is the stage from which sleep terrors and sleepwalking are most likely to occur. Stage 4 is also the stage, it's thought, when the body does most of its repair work.
REM Sleep
We name this stage after the most easily seen physical marker: rapid eye movements. When a person is in the REM stage, he or she will often show bursts of eye flutters and back-and-forth movements, possibly in conjunction with dreams. This is the stage when the most vivid and prolonged dreaming occurs. Your brain is highly active (on sleep graphs, it looks almost the same as your brain activity when you're awake), but your body is temporarily paralyzed by an active, generalized inhibition originating in the lower brain. Your muscles—even the ancillary respiratory muscles other than the diaphragm—are "shut down" and completely inert, and it's believed that this is to protect us from acting out our dreams. Your heart rate picks up; blood flow to the brain increases; respiration becomes erratic, faster, and shallower, with increased oxygen consumption; metabolism increases; sexual arousal occurs, with men developing erections and women clitoral engorgement; and blood pressure rises. Your brain activity shows low-voltage, fast-frequency waves. You also lose some of your ability to regulate your body temperature during this stage, so you're more susceptible to the temperature of your surroundings.
REM sleep is thought to be especially important in learning and memory processing, but there is much debate still about what exactly takes place. The ease of arousing someone from REM sleep varies, but it's usually far easier to wake someone from REM sleep than from stage 3 or 4 sleep. You can also become alert much more quickly when someone wakes you out of REM sleep than when you are aroused out of delta NREM sleep. About 20 to 25 percent of an adult's total sleep time in spent in the REM stage.
THE CYCLE
You might think that the sleep cycle would progress as stages 1, 2, 3, and 4, then REM, then repeat, but it often doesn't occur in quite such a regular manner. Instead, a typical adult sleep cycle looks more like the following.
Stage 1 for up to 10 minutes, stage 2 for 10 to 15 minutes, stage 3 for about 5 minutes, stage 4 for 20 to 40 minutes, then back to stage 3 for just a minute or two, stage 2 for 5 to 10 minutes, then REM for up to 5 minutes.
The first REM stage begins about 90 minutes into sleep, then the cycle starts again about every 90 minutes throughout the night.
During the earlier cycles, stages 3 and 4 are at their longest, but they get shorter and may drop out altogether in later cycles, with stage 2 lengthening to fill in the gap. The REM stage gets longer in the later cycles. In total, you'll spend about 75 to 80 percent of the night in NREM sleep and 20 to 25 percent in REM sleep.
The pr...
