The late biopsychologist John Gibbon called time the primordial context: a fact of life that has been felt by all organisms in every era. For the morning glory that spreads its petals at dawn, for geese flying south in autumn, for locusts swarming every 17 years and even for lowly slime molds sporing in daily cycles, timing is everything. In human bodies, biological clocks keep track of seconds, minutes, days, months and years. They govern the split-second moves of a tennis serve and account for the trauma of jet lag, monthly surges of menstrual hormones and bouts of wintertime blues. Cellular chronometers may even decide when your time is up. Life ticks, then you die.
The pacemakers involved are as different as stopwatches and sundials. Some are accurate and inflexible, others less reliable but subject to conscious control. Some are set by planetary cycles, others by molecular ones. They are essential to the most sophisticated tasks the brain and body perform. And timing mechanisms offer insights into aging and disease. Cancer, Parkinson's disease, seasonal depression and attention-deficit disorder have all been linked to defects in biological clocks.
The physiology of these timepieces is not completely understood. But neurologists and other clock researchers have begun to answer some of the most pressing questions raised by human experience in the fourth dimension. Why, for example, a watched pot never boils. Why time flies when you're having fun. Why allnighters can give you indigestion, and why people live longer than hamsters. It's only a matter of time before clock studies resolve even more profound quandaries of temporal existence.
The Psychoactive Stopwatch
IF THIS ARTICLE intrigues you, the time you spend reading it will pass quickly. It'll drag if you get bored. That's a quirk of a stopwatch in the brain--the so-called interval timer--that marks time spans of seconds to hours. The interval timer helps you figure out how fast you have to run to catch a baseball. It tells you when to clap to your favorite song. It lets you sense how long you can lounge in bed after the alarm goes off.
Interval timing enlists the higher cognitive powers of the cerebral cortex, the brain center that governs perception, memory and conscious thought. When you approach a yellow traffic light, for example, you time how long it has been yellow and compare that with a memory of how long yellow lights usually last. then you have to make a judgment about whether to put on the brakes or keep driving, says Stephen M. Rao of the Medical College of Wisconsin.
Rao's studies with functional magnetic resonance imaging (fMRI) have pointed to the parts of the brain engaged in each of those stages. In the fMRI machine, subjects listen to two pairs of tones and decide whether the interval between the second pair is shorter or longer than the interval between the first. The brain structures that are involved in the task consume more oxygen than those that are not involved, and the fMRI scan records changes in blood flow and oxygenation once every 250 milliseconds. When we do this, the very first structures that are activated are the basal ganglia, Rao says.
Long associated with movement, this collection of brain regions has become a prime suspect in the search for the interval-timing mechanism as well. One area of the basal ganglia, the striatum, hosts a population of conspicuously well-connected nerve cells that receive signals from other parts of the brain. The long arms of these striatal cells are covered with between 10,000 and 30,000 spines, each of which gathers information from a different neuron in another locale. If the brain acts like a network, then the striatal spiny neurons are critical nodes. This is one of only a few places in the brain where you see thousands of neurons converge on a single neuron, says Warren H. Meck of Duke University.
Striatal spiny neurons are central to an interval-timing theory Meck developed over the past decade with Gibbon, who worked at Columbia University until his death in 2001. The theory posits a collection of neural oscillators in the cerebral cortex: nerves cells firing at different rates, without regard to their neighbors' tempos. In fact, many cortical cells are known to fire at rates between 10 and 40 cycles per second without external provocation. All these neurons are oscillating on their own schedules, Meck says, like people talking in a crowd. None of them are synchronized.
The cortical oscillators connect to the striatum via millions of signal-carrying arms, so the striatal spiny neurons can eavesdrop on all those haphazard conversations. Then something--a yellow traffic light, say--gets the cortical cells' attention. The stimulation prompts all the neurons in the cortex to fire simultaneously, causing a characteristic spike in electrical output some 300 milliseconds later. This attentional spike acts like a starting gun, after which the cortical cells resume their disorderly oscillations.
But because they have begun simultaneously, the cycles now make a distinct, reproducible pattern of nerve activation from moment to moment. The spiny neurons monitor those patterns, which help them to count elapsed time. At the end of a specified interval--when, for example, the traffic light turns red--a part of the basal ganglia called the substantia nigra sends a burst of the neurotransmitter dopamine to the striatum. The dopamine burst induces the spiny neurons to record the pattern of cortical oscillations they receive at that instant, like a flashbulb exposing the interval's cortical signature on the spiny neurons' film. There's a unique time stamp for every interval you can imagine, Meck says.
Once a spiny neuron has learned the time stamp of the interval for a given event, subsequent occurrences of the event prompt both the firing of the cortical starting gun and a burst of dopamine at the beginning of the interval [see top illustration in box on opposite page]. The dopamine burst now tells the spiny neurons to start tracking the patterns of cortical impulses that follow. When the spiny neurons recognize the time stamp marking the end of the interval, they send an electrical pulse from the striatum to another brain center called the thalamus. The thalamus, in turn, communicates with the cortex, and the higher cognitive functions--such as memory and decision making--take over. Hence, the timing mechanism loops from the cortex to the striatum to the thalamus and back to the cortex again.
If Meck is right and dopamine bursts play an important role in framing a time interval, then diseases and drugs that affect dopamine levels should also disrupt that loop. So far that is what Meck and others have found. Patients with untreated Parkinson's disease, for example, release less dopamine into the striatum, and their clocks run slow. In trials these patients consistently underestimate the duration of time intervals. Marijuana also lowers dopamine availability and slows time. Recreational stimulants such as cocaine and methamphetamine increase the availability of dopamine and make the interval clock speed up, so that time seems to expand. Adrenaline and other stress hormones make the clock speed up, too, which may be why a second can feel like an hour during unpleasant situations. States of deep concentration or extreme emotion may flood the system or bypass it altogether; in such cases, time may seem to stand still or not exist at all. Because an attentional spike initiates the timing process, Meck thinks people with attention-deficit hyperactivity disorder might also have problems gauging the true length of intervals.
The interval clock can also be trained to greater precision. Musicians and athletes know that practice improves their timing; ordinary folk can rely on tricks such as chronometric counting (one one-thousand) to make up for the mechanism's deficits. Rao forbids his subjects from counting in experiments because it could activate brain centers related to language as well as timing. But counting works, he says--well enough to expose cheaters. The effect is so dramatic that we can tell whether they're counting or timing based just on the accuracy of their responses.
The Somatic Sundial
ONE OF THE VIRTUES of the interval-timing stopwatch is its flexibility. You can start and stop it at will or ignore it completely. It can work subliminally or submit to conscious control. But it won't win any prizes for accuracy. The precision of interval timers has been found to range from 5 to 60 percent. They don't work too well if you're distracted or tense. And timing errors get worse as an interval gets longer. Hence the instruments we all wear on our wrists, Rao notes.
Fortunately, a more rigorous timepiece chimes in at intervals of 24 hours. The circadian clock--from the Latin circa (about) and diem (a day)--tunes our bodies to the cycles of sunlight and darkness caused by the earth's rotation. It helps to program the daily habit of sleeping at night and waking in the morning. But its influence extends much further. Body temperature regularly peaks in the late afternoon or early evening and bottoms out a few hours before we rise in the morning. Blood pressure typically starts to surge between 6:00 and 7:00 A.M. Secretion of the stress hormone cortisol is 10 to 20 times higher in the morning than at night. Urination and bowel movements are generally suppressed at night and pick up again in the morning.
The circadian timepiece is more like a clock than a stopwatch because it runs without the need for a stimulus from the external environment. Studies of volunteer cave dwellers and other human guinea pigs have demonstrated that circadian patterns persist even in the absence of daylight, occupational demands and caffeine. And they are expressed in every cell of the body. Confined to a petri dish under constant lighting, human cells still follow 24-hour cycles of gene activity, hormone secretion and energy production. The cycles are hardwired, and they vary by as little as 1 percent: just minutes a day.
But if light isn't required to establish a circadian cycle, it is needed to synchronize the phase of the hardwired clock with natural day and night cycles. Like an ordinary clock that runs a few minutes slow or fast each day, the circadian clock needs to be continually reset to stay accurate. Neurologists have made great progress in understanding how daylight sets the clock. Two clusters of 10,000 nerve cells in the hypothalamus of the brain have long been considered the clock's locus. Decades of animal studies have demonstrated that these centers, each called a suprachiasmatic nucleus (SCN), drive daily fluctuations in blood pressure, body temperature, activity level and alertness. The SCN also tells the brain's pineal gland when to release melatonin, which promotes sleep in humans and is secreted only at night.
In 2002 separate teams of researchers proved that dedicated cells in the retina of the eye transmit information about light levels to the SCN. These cells--a subset of those known as ganglion cells--operate completely independently of the rods and cones that mediate vision, and they are far less responsive to sudden changes in light. That sluggishness befits a circadian system. It would be no good if watching fireworks or going to a movie matinee tripped the mechanism.
But the SCN's role in circadian rhythms is being reevaluated in view of other findings. Scientists had assumed that the SCN somehow coordinated all the individual cellular clocks in the body's organs and tissues. Then, in the mid-1990s, researchers discovered four critical genes that govern circadian cycles in flies, mice and humans. These genes turned up not just in the SCN but everywhere else, too. These clock genes are expressed throughout the whole body, in every tissue, says Joseph Takahashi of North western University. We didn't expect that.
And in 2002 researchers at Harvard University reported that the expression of more than 1,000 genes in the heart and liver tissue of mice varied in regular 24-hour periods. But the genes that showed these circadian cycles differed in the two tissues, and their expression peaked in the heart at different hours than in the liver. They're all over the map, says Michael Menaker of the University of Virginia. Some are peaking at night, some in the morning and some in the daytime.
Menaker has shown that specific feeding schedules can shift the phase of the liver's circadian clock, overriding the light-dark rhythm followed by the SCN. When lab rats that usually ate at will were fed just once a day, for example, peak expression of a clock gene in the liver shifted by 12 hours, whereas the same clock gene in the SCN stayed locked in sync with light schedules. It makes sense that daily rhythms in feeding would affect the liver, given its role in digestion. Researchers think circadian clocks in other organs and tissues may respond to other external cues-- including stress, exercise, and temperature changes--that occur regularly every 24 hours. No one is ready to dethrone the SCN: its authority over body temperature, blood pressure and other core rhythms is still secure. But this brain center is no longer thought to rule the peripheral clocks with an iron fist. We have oscillators in our organs that can function independently of our oscillators in our brain, Takahashi says.
The autonomy of the peripheral clocks makes a phenomenon such as jet lag far more comprehensible. Whereas the interval timer, like a stopwatch, can be reset in an instant, circadian rhythms take days and sometimes weeks to adjust to a sudden shift in day length or time zone. A new schedule of light will slowly reset the SCN clock. But the other clocks may not follow its lead. The body is not only lagging; it's lagging at a dozen different paces.
Jet lag doesn't last, presumably because all those different drummers are able to eventually sync up again. But shift workers, party animals, college students and other night owls face a worse chro no dilemma. They may be leading a kind of physiological double life. Even if they get plenty of shut-eye by day, their core rhythms are still ruled by the SCN--hence, the core functions continue sleeping at night. You can will your sleep cycle earlier or later, says Alfred J. Lewy of the Oregon Health & Science University. But you can't will your melatonin levels earlier or later, or your cortisol levels, or your body temperature.
Meanwhile their schedules for eating and exercising could be setting their peripheral clocks to entirely different phases from either the sleep-wake cycle or the light-dark cycle. With their bodies living in so many time zones at once, it's no wonder shift workers have an increased incidence of heart disease, gas-gastrointestinal complaints and, of course, sleep disorders.
A Clock for All Seasons
JET LAG AND SHIFT work are exceptional conditions in which the innate circadian clock is abruptly thrown out of phase with the light-dark cycles or sleep-wake cycles. But the same thing can happen every year, albeit less abruptly, when the seasons change. Research shows that although bedtimes may vary, people tend to get up at about the same time in the morning year-round--usually because their dogs, kids, parents or careers demand it. In the winter, at northern latitudes, that means many people wake up two to three hours before dawn. Their sleepwake cycle is several time zones away from the cues they get from daylight.
The mismatch between day length and daily life could explain the syndrome known as seasonal affective disorder, or SAD. In the U.S., SAD afflicts as many as one in 20 adults with depressive symptoms such as weight gain, apathy and fatigue between October and March. The condition is 10 times more common in the north than the south. Although SAD occurs seasonally, some experts suspect it is actually a circadian problem. Lewy's work suggests that SAD patients would come out of their depression if they could get up at the natural dawn in the winter. In his view, SAD is not so much a pathology as evidence of an adaptive, seasonal rhythm in sleep-wake cycles. If we adjusted our daily schedules according to the seasons, we might not have seasonal depression, Lewy says. We got into trouble when we stopped going to bed at dusk and getting up at dawn.
If modern civilization doesn't honor seasonal rhythms, it's partly because human beings are among the least seasonally sensitive creatures around. SAD is nothing compared to the annual cycles other animals go through: hibernation, migration, molting and especially mating, the master metronome to which all other seasonal cycles keep time. It is possible that these seasonal cycles may also be regulated by the circadian clock, which is equipped to keep track of the length of days and nights. Darkness, as detected by the SCN and the pineal gland, prolongs melatonin signals in the long nights of winter and reduces them in the summer. Hamsters can tell the difference between a 12-hour day, when their gonads don't grow, and a 12-hour-15-minute day, when their gonads do grow, Menaker says [see box below].
If seasonal rhythms are so robust in other animals, and if humans have the equipment to express them, then how did we ever lose them? What makes you think we ever had them? Menaker asks. We evolved in the tropics. Menaker's point is that many tropical animals don't exhibit dramatic patterns of annual behavior. They don't need them, because the seasons themselves vary so little. Most tropical animals mate without regard to seasons because there is no best time to give birth. People, too, are always in heat. As our ancestors gained greater control of their environment over the millennia, seasons probably became an even less significant evolutionary force.
But one aspect of human fertility is cyclical: women and other female primates produce eggs just once a month. The clock that regulates ovulation and menstruation is a well-documented chemical feedback loop that can be manipulated by hormone treatments, exercise and even the presence of other menstruating women. But the reason for the specific duration of the menstrual cycle is unknown. The fact that it is the same length as the lunar cycle is a coincidence few scientists have bothered to investigate, let alone explain. No convincing link has yet been found between the moon's radiant or gravitational energy and a woman's reproductive hormones. In that regard, the monthly menstrual clock remains a mystery--outdone perhaps only by the ultimate conundrum, mortality.
Time the Avenger
PEOPLE TEND TO EQUATE aging with the diseases of aging--cancer, heart disease, osteoporosis, arthritis and Alzheimer's, to name a few--as if the absence of disease would be enough to confer immortality. Biology suggests otherwise.
Modern humans in developed countries have a life expectancy of more than 70 years. The life expectancy of your average mayfly, in contrast, is a day. Biologists are just beginning to explore why different species have different life expectancies. If your days are numbered, what's doing the counting?
At a 2002 meeting hosted by the National Institute on Aging, participants challenged many common assumptions about the factors that determine natural life span. The answer cannot lie solely with a species' genetics: worker honeybees, for example, last a few months, whereas queen bees live for years. But genetics are important: a single-gene mutation in mice can produce a strain that lives up to 50 percent longer than usual. High metabolic rates can shorten life span, yet many species of birds, which have fast metabolisms, live longer than mammals of comparable body size. And big, slow-metabolizing animals do not necessarily outlast the small ones. The life expectancy of a parrot is about the same as a human's. Among dog species, small breeds typically live longer than large ones.
Scientists in search of the limits to human life span have traditionally approached the subject from the cellular level rather than considering whole organisms. So far the closest thing they have to a terminal timepiece is the socalled mitotic clock. The clock keeps track of cell division, or mitosis, the process by which a single cell splits into two. The mitotic clock is like an hourglass in which each grain of sand represents one episode of cell division. Just as there is a finite number of grains in an hourglass, there seems to be a ceiling on how many times normal cells of the human body can divide. In culture they will undergo 60 to 100 mitotic divisions, then call it quits. All of a sudden they just stop growing, says John Sedivy of Brown University. They respire, they metabolize, they move, but they will never divide again.
Cultured cells usually reach this state of senescence in a few months. Fortunately, most cells in the body divide much, much more slowly than cultured cells. But eventually--perhaps after 70 years or so--they, too, can get put out to pasture. What the cells are counting is not chronological time, Sedivy says. It's the number of cell divisions.
In the late 1990s Sedivy reported that he could squeeze 20 to 30 more cycles out of human fibroblasts by mutating a single gene. This gene encodes a protein called p21, which responds to changes in structures called telomeres that cap the end of chromosomes. Telomeres are made of the same stuff that genes are: DNA. They consist of thousands of repetitions of a six-base DNA sequence that does not code for any known protein. Each time a cell divides, chunks of its telomeres are lost. Young human embryos have telomeres between 18,000 and 20,000 bases long. By the time senescence kicks in, the telomeres are only 6,000 to 8,000 bases long.
Biologists suspect that cells become senescent when telomeres shrink below some specific length. Titia de Lange of the Rockefeller University has proposed a new explanation for this link. In healthy cells, she showed, the chromosome ends are looped back on themselves like a hand tucked in a pocket. The hand is the last 100 to 200 bases of the telomere, which are single-stranded, not paired like the rest. With the help of more than a dozen specialized proteins, the single-stranded end is inserted into the double strands upstream for protection.
If telomeres are allowed to shrink enough, they can no longer do this looping trick, de Lange says. Untucked, a single-stranded telomere end is vulnerable to fusion with other single-stranded ends. The fusion wreaks havoc in a cell by stringing together all the chromosomes. That could be why Sedivy's mutated p21 cells died after they got in their extra rounds of mitosis. Other cells bred to ignore short telomeres have turned cancerous. The job of normal p21 and telomeres themselves may be to stop cells from dividing so much that they die or become malignant. Cellular senescence could actually be prolonging human life rather than spelling its doom. It might be cells' imperfect defense against malignant growth and certain death.
Our hope is that we'll gain enough information from this reductionist approach to help us understand what's going on in the whole person, de Lange comments.
For now, the link between shortened telomeres and aging is tenuous at best. Most cells do not need to keep dividing to do their job--white blood cells that fight infection and sperm precursors being obvious exceptions. But many older people do die of simple infections that a younger body could withstand. Senescence probably has nothing to do with the nervous system, Sedivy says, because most nerve cells do not divide. On the other hand, it might very well have something to do with the aging of the immune system.
In any case, telomere loss is just one of the numerous insults cells sustain when they divide, says Judith Campisi of Lawrence Berkeley National Laboratory. DNA often gets damaged when it is replicated during cell division, so cells that have split many times are more likely to harbor genetic errors than young cells. Genes related to aging in animals and people often code for proteins that prevent or repair those mistakes. And with each mitotic episode, the by-products of copying DNA build up in cell nuclei, complicating subsequent bouts of replication.
Cell division is very risky business, Campisi observes. So perhaps it is not surprising that the body puts a cap on mitosis. And cheating cell senescence probably wouldn't grant immortality. Once the grains of sand have fallen through the mitotic hourglass, there's no point in turning it over again.