Body clocks.

Try this: For an entire day, forget about the clock. Eat when you're hungry and sleep when you're tired. What do you think will happen?

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You may be surprised to find that your day is much like most other days. You'll probably get hungry when you normally eat and tired when you normally sleep. Even though you don't know what time it is, your body does.

These patterns of daily life are called circadian rhythms, and they are more than just habits. Inside our bodies are several clocklike systems that follow a roughly 24-hour cycle. Throughout the day and night, our internal clocks direct changes in temperature, body chemicals, hunger, sleepiness and more.

Everyone's rhythms are unique, which is why you might like to stay up late while your sister always wants to go to bed early. But overall, everyone is programmed to feel tired at night and alert during the day.

Scientists have known for a long time that the light of day and the dark of night play important roles in setting our internal clocks. Now, new discoveries are giving scientists insights into how these clocks work.

Learning about our body clocks may help scientists understand why problems arise when we act out of step with our circadian rhythms. For example, traveling across time zones can make people wake up in the middle of the night. Regularly staying up late can make kids do worse on tests and quizzes. And working shifts at night leads to higher rates of heart disease, diabetes and obesity.

"There is a growing sense that when we eat and when we sleep are important parts of how healthy we are," says Steven Shea, Director of the Sleep Disorders Research Program at Brigham and Women's Hospital in Boston.

Scientists still aren't sure why the timing of sleep matters so much, Shea says. But research findings suggest that our circadian rhythms are more important than we give them credit for.

"During the night, we are prepared to sleep," Shea says. "During the day, we are prepared to eat and move around. If you reverse what you are doing, everything is out of phase. That can have adverse consequences."

Time warp 

One way to learn about how our body clocks tick is to mess them up and see what happens. That's what neuroscientist Frank Scheer and his colleagues did in a recent study.

The researchers brought 10 people to their lab at Harvard Medical School in Boston. The lab was sort of like a timeless chamber. Rooms were dimly lit. There were no windows and no clocks. It was impossible to know what time it was.

"If you knew it was 4 a.m., you'd think, 'I must be really tired,' " says Scheer, who also works with Shea at Brigham and Women's Hospital. Removing time cues eliminated these powers of suggestion.

Participants were allowed to sleep only when the scientists said it was OK. The study subjects ate only at designated mealtimes. They were given a precisely calculated number of calories, designed to meet their needs. And they had to finish everything on their plates.

The experiment lasted for 10 days. Participants didn't know the design of the experiment. In particular, they didn't know that they were living a 28-hour day instead of the usual 24. With that unusual schedule, they ended up eating and sleeping at all different times of day--and different times of the body clock--over the course of the study.

The most interesting result of the study, Scheer says, involved a hormone called leptin. Hormones are the body's messenger molecules. Leptin, in particular, sends a fullness message to the brain. As you eat, leptin levels rise until you feel like you've eaten enough.

When people in the study slept during the day and ate at night, however, leptin levels dropped. That suggests that people who follow unusual schedules are less likely to feel full after eating.

If given unlimited amounts of food, these people would probably eat more and crave more junk food, the researchers predict. As a result, they could gain weight and develop weight-related health problems, such as diabetes and heart disease. Other studies support that prediction.

Kids don't often work night shifts. "But some may experience staying up late at night," Scheer says. That's OK on special occasions.

But staying up night after night, these studies suggest, could make kids extra hungry and more likely to gain weight. And regularly sleeping too little, Scheer says, may be one cause of the recent surge in childhood obesity.

Eat to sleep

Scheer's work suggests that our sleeping schedules affect our eating habits. But do our eating schedules influence our sleeping habits?

New research suggests that it works both ways, says Clifford Saper. He's a neurologist, or a scientist who studies the brain, at Beth Israel Deaconess Medical Center in Boston.

In one recent study, Saper and colleagues investigated a different type of body clock in mice. Like people, mice have more than one internal clock. In the brain, there is the master clock that responds to light and helps determine when we get tired. There are a bunch of minor clocks, too, which reside in the gut, blood vessels and other parts of the body.

The master clock works like "the conductor of a symphony," Saper says. It's like the clock at school that determines when classes end and when lunch begins. Everyone sets their watches to this clock. In the body, the secondary clocks follow the lead of the master pacemaker.

Sometimes, however, the master clock gives up control. One example is when mice don't eat for a long time. If a food source appears when a hungry mouse is normally sleeping, and the mouse happens to wake up in time to find it, this minor food clock wakes the mouse up a couple of hours before that time, night after night.

This ability to change their schedules instantly helps mice survive. If the animals are starving, the food clock ensures that they are awake when food is available, even if it's an odd time to be up and an odd time to eat. It doesn't matter whether it's dark or light outside.

"The amazing thing is that the [food] clock ... adjusts to whatever time it finds the food immediately," Saper says. "It could make a 12-hour time shift overnight."

The master clock, on the other hand, can only adjust slowly to changes in light.

Saper wanted to know more about this food clock. In his study, he turned off all the internal clocks in a group of mice. Then, he turned the clocks back on, one by one.

His results pinpointed the food clock to a certain part of the mouse's brain. That's interesting because the master clock resides in a different part of the brain. Figuring out where the food clock is will help scientists better understand how it works.

Similar studies haven't been done in people, but human brains are wired much like mouse brains, Saper says. He suspects that people have a food clock, too. If so, his work might eventually help night shift workers learn to reset their circadian rhythms without health problems.

The mouse study might also offer help for people who suffer from jet lag when traveling. Jet lag is the exhaustion and disorientation that comes with crossing many time zones.

The master clock requires a day for every time zone crossed to adjust to the new time. But Saper's work suggests that people could speed up this process by jump-starting their food clocks. To do this, travelers would need to fast for at least 16 hours before eating breakfast at the normal time they would in their new destinations.

"You could potentially turn on the food clock and adjust to a new time zone very rapidly," Saper says. For now, he adds, "It's all speculation."