Your Body's Internal Clock and How It Affects Your Overall Health

生物钟是如何影响健康的

Your Body's Internal Clock and How It Affects Your Overall Health
By Alice G. Walton
Not getting enough sleep leaves us tired during the day. But the body's clock also affects mood, mental alertness, hunger, and heart function.
We all feel the ebb and flow of daily life, the daily rhythms that shape our days. The most basic daily rhythm we live by is the sleep-wake cycle, which (for most) is related to the cycle of the sun. It makes us feel sleepy as the evening hours wear on, and wakeful as the day begins. Sleep-wake and other daily patterns are part of our circadian rhythms, (circum means "around" and dies, "day") which are governed by the body's internal or biological clock, housed deep within the brain. 
But research has been finding that the body's clock is responsible for more than just sleep and wakefulness. Other systems, like hunger, mental alertness, and mood, stress, heart function, and immunity also operate on a daily rhythm.
The existence of the biological clock can be particularly apparent when it's off kilter: Jet lag and shift work can throw our normal patterns out of whack and take a toll on physical and mental health. Even shifting the clock an hour forward or backward when daylight savings time begins or ends can disrupt our biological clocks.
Disrupting our body's natural cycles can cause problems. Studies have found there are more frequent traffic accidents and workplace injuries when we spring forward and lose an hour of sleep. Heart patients are at greater risk for myocardial infarction in the week following the Daylight Savings time shift. But even more significant is that science continues to discover important connections between a disrupted clock and chronic health issues, from diabetes to heart disease to cognitive decline. 
It turns out that the same genes and biological factors that govern our internal clock are also involved in how other body systems operate — and break down. It can be hard to determine whether a disrupted clock leads to health problems, or whether it's the other way around. 
We're beginning to understand more about how the clock interacts with and helps govern the function of other systems and affects our overall health. In fact, keeping your body's daily cycle on an even keel may be one of the best things you can do for your overall health. 
YOUR BODY WANTS TO RUN LIKE A SWISS WATCH
The idea of a biological clock may sound like a quaint metaphor, but there is actually a very distinct brain region that is charged with keeping time: It is an area called the suprachiasmatic nucleus (or SCN), situated right above the point in the brain where the optic nerve fibers cross. This location enables the SCN to receive the cues it needs from light in the environment to help it keep time. 
But genes also influence the body's clock and circadian rhythms. The system requires both types of input — light and genes — to keep it on track. To stay on the 24-hour cycle, the brain needs the input of sunlight through the eyes to reset itself each day. When humans are allowed to run off their body's clock apart from input from the sun, by being kept in continuous darkness, the body's daily cycle tends to lengthen to about 25 hours. And when people or animals lack the genes that help control the clock's cycle, their sleep-wake cycles can stray even further, or be absent completely. The need for both kinds of cues — light and genes — make the biological clock a classic example of how genes and the environment work in tandem to keep the system functioning well. 
Our Behaviors and Body Functions Run on Cycle
Melatonin is one hormone responsible for our body's daily cycle. When night falls and there is less light input to the SCN, the production of melatonin, the hormone responsible for making us feel sleepy, goes up. When it's dark, more melatonin is secreted, which signals the brain to go into sleep mode. When the sun rises, melatonin secretion is inhibited, and the brain's awake circuits resume. 
Other systems also follow a daily rhythm, many of which are controlled by hormones and other compounds that receive cues from the biological clock. For example, the hormones responsible for hunger and metabolism rise and fall over the course of the day. The chemicals involved in immune system function also vary. Compounds that encourage the inflammatory response rise at night, (which is why fevers tend to spike then), and those that inhibit it rise during the day.
This is likely because the body is better at fighting infection while it is at rest, and energy can be poured into the effort, rather than into other functions. And activity of the stress response system — particularly in secretion of the stress hormone, cortisol — is reduced during the nighttime hours, and heightened in the early morning.
Although there are certain areas of the body, like the heart, that are able to govern their own function to some degree, there is strong evidence that the body clock plays a major role in controlling many of these fluctuations (such as in blood sugar) over the 24-hour period.
ENVIRONMENTAL DISRUPTIONS TO THE BODY'S CLOCK
Some of the best knowledge we have about the roles the biological clock plays in our health come from instances in which the cycle gets out of sync. This can happen for different reasons, and we're just starting to understand them in greater detail. Sometimes we do things ourselves that disrupt our normal rhythms, like flying to a distant time zone. Sometimes it's other factors, (like genes or biology) that play a role. 
Jet Lag
Flying across the country on the red-eye is a prime example of how we can disrupt our own clocks, and a far more extreme example than the spring forward/fall back ritual in many parts of the U.S.
When jet lag sets in, we feel disoriented, foggy, and sleepy at the wrong times of day because, after changing time zones, our body clock tells us it's one time and the outside environment tells us it's another. In fact, jet lag can be considered one type of circadian rhythm disorder. It can be treated simply be allowing the body to adjust to the new time, although it may take several days for external cues (light) to help the internal clock catch up or fall back with its new cycle. 
Shift Work
Shift work is another example of how we can get ourselves off-cycle, and this too can develop into a circadian rhythm disorder over the long term. People who work the night shift not only have a hard time with their sleep patterns (feeling sleepy at work or experiencing insomnia during the day), but other systems in their bodies can also feel the effects — and they can be chronic. It's not been clear exactly why this connection exists, but weight gain or metabolic changes may be involved. These phenomena underline how particular behaviors or lifestyles can affect the body's clock, but there are other factors at play, like genetics and body chemistry.
BIOLOGICAL AND GENETIC DISRUPTIONS AND THEIR IMPLICATIONS FOR HEALTH
The interactions of the clock are complex, and their effects on different body systems are intricate, but we're starting to understand more about how the nuts and bolts of the clock work, and affect each system of the body, from our hearts to our moods. 
Since the biological clock is, in fact, a biological entity, things can go wrong with it that may have less to do with lifestyle or the environment, and more to do with the mechanisms of the clock itself. For example, there's more to the clock-diabetes link than just turning our sleep cycle around, though sleep can make a difference. 
The same genes that control the receptors for the sleep hormone melatonin are involved in insulin release, which could also play a role in diabetes risk. When melatonin receptor genes have mutations that damage the connection between the biological clock and insulin release people have a significantly higher risk of developing diabetes. 
The Rhythms of the Heart
The heart is one organ that, although it can keep time by itself to some degree, relies on the brain's biological clock for cues. For years doctors and researchers have noticed that heart problems like fatal arrhythmias are more likely to occur at certain times of the day, both in the early morning and to a lesser degree, in the evening hours. Taking blood pressure medication in the evening seems to improve its effectiveness because it works with the body's circadian rhythms. 
The reason for this has recently become clear: A genetic factor involved in the rhythm of the brain's clock also controls the electrical activity in the heart. Mice who are bred to lack this factor — Kruppel-like factor 15 (KLF15) — or have too much of it, have many more heart problems than normal mice. Understanding this clock-heart connection could help experts design drugs to reduce the risk of heart problems in people by stabilizing the levels of these compounds.
Immunity and Vaccinations
Most of us have experienced being more susceptible to getting sick when sleep-deprived. The reason for this appears to be that certain chemicals responsible for immune function, like cytokines, wax and wane throughout the day and sleep deprivation deprives us of their best effects. Animals who are given vaccines at specific times of the day, when certain proteins that sense bacterial invaders are highest, have a much stronger immune response, even weeks later. The same is very likely true for humans. 
Body rhythms don't just enhance vaccines' ability to provide immunity; they can affect the body's ability to battle infection on its own. When mice were exposed to a bacterial infection, the severity of their infection reflected the time of day they were infected.
It's not just in the lab that these effects are seen. Babies who are given vaccines in the afternoon — and who sleep more right after — have better immune responses to the innoculations. It's likely that the same effect is true in adults, since our immune systems fluctuate in similar ways. 
Rhythm and Moods
Our internal clocks also have a hand in whether we feel up or down emotionally. People with mood disorders like depression, bipolar disorder, and seasonal affective disorder (SAD) have altered circadian rhythms. In fact, sleep disturbances, both sleeping too much and too little, are one of the key symptoms of depression and other mood disorders. 
The relationship between body rhythms and mood is an intricate one, and likely has to do with how the brain chemical serotonin fluctuates in relation to the light-dark cycle and throughout the year as the days become longer and shorter. Mice bred to have problems with serotonin function also have seriously altered daily rhythms. People's serotonin levels increase during the part of the day when there is more light available.
The circadian rhythm-mental health connection has also been linked to disease states like Alzheimer's, Parkinson's, and Huntington's, and even autism spectrum disorder. Researchers are finding that disrupted daily rhythms can be good predictors for the development of mild cognitive impairment that comes with age, and even for dementia.
Experiments in fruit flies (which may seem a far cry from humans, but actually serve as excellent models in biological clock studies) show that degeneration in the brain occurs much more rapidly when there are problems in the functioning of a key clock gene, and the lifespans of the flies are significantly shortened. Knowing more about how the clock is related to cognitive function and decline could help experts predict — and perhaps one day prevent — it from occurring in humans as well. 
KEEPING TIME
Paying attention to the body's natural rhythms is probably more important to our health than we realize. It's not just sleep deprivation that affects our well-being, but it's also the alteration of our biological rhythms that can interfere with so many body functions, making us more prone to health problems like infection, mood problems, and even heart disease. 
Why the biological clock becomes disrupted in certain people, or naturally with age, is not completely clear, but some have recently suggested that it could in part have to do with the aging of the eyes. Natural changes in the lens and even the development of cataracts let less light into the eye and, therefore, the brain; and this can affect biological rhythms.
There are many other reasons our bodies' clocks can go out of sync, which probably involve a combination of genetic predisposition and lifestyle choices, such as alcohol consumption. Sometimes the clock can get unset — as with the changes associated with daylight savings time, air travel, or shift work — and there's only so much we can do until our body and its clock are in equilibrium again. 
But keeping your schedule on track as much as possible is probably the best advice. You probably have a pretty good sense of your body's natural rhythms intuitively. Avoid disruptions to your eat-sleep cycles. Practice good sleep hygiene, and stick to a sleep schedule that works well for your body to keep the system in its natural rhythm. Turning in a little earlier, cutting back on caffeine late in the day, and saving that last bit of work for the morning rather than staying late up to finish it, can make a big difference in how your internal clock functions and in how you feel. 

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