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Sleep and Dreams: The Rhythm of Sleep

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Sleep and Dreams was originally published by Grolier Publishing in 2000.

When you sleep late on weekends and nobody tries to wake you up, you do get up, right? Of course you do. And later that night, perhaps at the end of your favorite television show, you head off to bed and probably go to sleep shortly afterward, right?

What causes you to wake up in the morning, even in a silent house with no interruptions? Why don’t you just sleep all morning and afternoon? Likewise, what causes you to feel drowsy in the evening and eventually drop off to sleep?

Believe it not, the answer to both questions is, the sun.

Deep inside your brain is a clump of nerve cells called the suprachiasmatic nuclei (SCN) that acts as a kind of body clock. It decides when it’s time to get up and when it’s time to go to sleep. The SCN resets itself each day, an action similar to resetting your alarm clock each day.

The resetting of the SCN depends on the regular appearance of sunlight and, later, the darkness that follows. Without the light of day and the darkness of night, the SCN can’t figure out how to regulate the body’s natural sleep-wake cycle.

Wake-up times and go-to-sleep times occur each day as part of the body’s natural sleep-wake cycle, one of several natural daily rhythms called circadian rhythms. A circadian rhythm is a series of events that occurs in the body once every twenty-four hours. Humans experience several circadian rhythms each day.

For instance, growth hormone levels rise and fall in a particular pattern throughout the day. The hormone reaches its peak shortly after you fall asleep, drops during the night, and stays low all day long before building back up again during sleep. Your body temperature also follows a natural cycle, peaking between 10:00 AM and 2:00 PM and dropping to a twenty-four-hour low in the middle of the night, at around 3:00 AM.

The sleep-wake cycle, one of the strongest circadian rhythms in the body, resets itself everyday in response to sunlight. A famous experiment carried out in 1938 showed the extremely strong effect sunlight has on circadian rhythms. Two sleep researchers, Nathaniel Kleitman and Bruce Richardson, spent thirty-three days in Mammoth Cave in Kentucky. During that time, they lost total track of time. The researchers awoke when their body told them to and fell asleep when they were sleepy. Soon, the researchers were falling asleep about an hour later than they normally would have if they were at the surface, exposed to the daily cycle of sunlight and darkness, sunlight and darkness. At the end of the first week, they were heading to bed about seven hours later than everyone else.

The results of the Mammoth Cave experiment, and of other experiments like it, led researchers to believe that the body’s sleep-wake cycle doesn’t match the twenty-four-hour clock we live our lives by. They thought the cycle was actually twenty-five to twenty-six hours long. Because a person’s natural sleep-wake cycle didn’t match the twenty-four-hour clock, the researchers said, it didn’t take much to disturb the ability to sleep at night.

Then, in early 1999, researchers in Boston determined that the body’s natural sleep-wake cycle wasn’t twenty-five or twenty-six hours long at all but only slightly longer than twenty-four hours—twenty-four hours and thirteen minutes, to be exact.

Dr. Charles Czeisler of Brigham and Women’s Hospital in Boston, along with sleep researchers at Harvard University in Cambridge, performed experiments similar to the ones performed by sleep researchers in the past. The difference was that the Boston researchers more closely controlled the amount of artificial light research subjects were exposed to. The results indicated that the SCN is far more sensitive to light than was once thought.

The SCN receives signals directly from a special chemical in the eye released when light falls on the eye. Light falling on the eye is made up of a wide range of colors, including reds, yellows, and blues. A special light-absorbing chemical called cryptochrome absorbs blue light and transfers chemical signals to the optic nerve, the main nerve leading from the eye to the brain. The optic nerve carries the signals to the SCN.

The SCN, in turn, tells the nearby pineal gland to produce a special sleep chemical called melatonin. Based on signals from the SCN, the pineal gland produces more melatonin or closes down production of the chemical.

Melatonin acts as a kind of messenger of sleep, traveling throughout the body and telling cells to slow their activities. The pineal gland releases hardly any melatonin during the day, a time during which body cells are usually running full force. As the sun sets, though, the pineal gland begins pouring out melatonin, causing cell activities to gradually slow as the body prepares for sleep. The surge in melatonin reaches its peak at around midnight.

Exposure to even small amounts of light in the late evening, when melatonin levels are high, can reduce the flow of melatonin nearly to daytime levels. This decrease in melatonin can throw off the body’s natural sleep-wake cycle. Teenagers are particularly affected by variations in melatonin levels.

At some point during the early- to mid-teen years, the pineal gland begins to release melatonin later in the evening than usual. As a result, teens find they can stay up later at night without feeling tired. Sleep often doesn’t arrive until after midnight.

Because many high schools start classes early in the morning, teenagers may need to awaken before they’ve had enough sleep. It’s as if a basic miscommunication exists between the teen’s bedside alarm clock, buzzing out a wake-up signal, and the teen’s internal body clock, which says, "Hey, you’ve got two more hours of sleep!" As a result of this miscommunication, it may take a teen’s body several hours before his body clock catches up with his wristwatch.

Some school districts in the United States understand this miscommunication and delay starting times for high school classes. A later starting time allows teens to sleep later and gain the rest they need to perform their best.

Light is absolutely critical for resetting the body clock. Many people who are blind are highly affected by the absence of light and, as a result, often experience sleep problems. Exposure to bright light in the morning can reset the body clock by as much as ninety minutes a day.

That means that if you stay up, say, two hours past your normal bedtime on Friday night, and another hour late on Saturday night, your body clock will end up being a total of three hours behind. Exposing yourself to bright light for two mornings in a row can reset your body clock right back to where it belongs.

The body clock can also act as a kind of internal alarm clock when you really need to wake up on time. Maybe you need to leave early for a bus trip to your school’s championship game or to fly to Florida for a vacation. You’re anxious that you might not wake up in time. Maybe the alarm clock will fail. Maybe you’ll turn the alarm off in your sleep.

In such instances, a special chemical called adrenocorticotropic hormone, or ACTH, sets a kind of alarm clock in the brain. Normally ACTH and other chemicals help the body respond to stress or prepare for stressful events. For instance, when you’re facing an important test you haven’t studied enough for, the level of ACTH in your bloodstream skyrockets. When the stressful event has passed, the ACTH level drops back to normal.

Researchers at the University of Texas Medical Branch in Galveston found that ACTH levels rise during sleep just as they do during the day. The researchers monitored ACTH levels in fifteen volunteers at various times during the night and in the early morning. The tests showed that when the volunteers planned to wake up at a certain time, ACTH levels shot up quickly about an hour or so before the planned wake-up time. When the volunteers made no plans to wake up early, however, ACTH levels rose much more slowly.

In a way, the brain reads its own clock and then, well before the real alarm clock buzzes, tells the body to pour out ACTH. "OK, body," the brain seems to be saying, "get ready to wake up!"

No matter what kind of clock you use to wake up—internal, electric, or wind-up—you probably tend to be most alert either in the morning or the evening. If you’ve always been one of those people who is most alert in the morning and becomes less and less alert as the day wears on, you’re a morning person. Sleep researchers would call you a lark, named after the meadowlark, a songbird that sings to the sunrise.

On the other hand, if you’ve always been one of those people who is most alert in the evening and who enjoys staying up late at night, you’re an evening person. Sleep researchers would call you an owl, in recognition of a bird active only at night.

Most people have characteristics of both larks and owls. One of the key differences between larks and owls relates to peaks and dips in body temperature. Body temperature normally varies by as much as one degree Celsius over each twenty-four-hour period. The body temperature of a lark tends to peak earlier in the day than does the temperature of an owl. With that peak body temperature usually come peak levels of alertness.

In addition, body temperature in a lark tends to rise quickly in the morning and then stays relatively steady until the early evening. Then, as the evening wears on, the lark’s body temperature takes a nose dive. Soon, drowsiness sets in and the person feels ready for bed.

Body temperature in owls, on the other hand, rises more slowly in the morning and reaches a peak in the evening. Then, during the late evening, the owl’s body temperature finally begins to fall.

Whether you’re a lark or a night owl depends almost solely on the traits your parents passed on to you, and that their parents had passed on to them. In other words, once a lark, always a lark, and once an owl, always an owl.

However, knowing the time of day you’re most alert may help you adjust your lifestyle to fit your own pattern of alertness. For instance, if you know you’re mostly a lark, you might want to study for a test in the afternoon rather than in the evening. If you’re a night owl, you might want to study in the evening rather than the morning or afternoon.