Inside every one of us is a clock, a tiny cluster of 20,000 cells in the frontal part of the brain. This natural chronometer, set by morning light hitting the retina, establishes our roughly 24—hour biological pace, also known as circadian rhythm. The clock then sends a signal to most body organs: It's time to wake up. When the lights go off at night, it preps the body for sleep.
Rooted in Earth's light—dark cycles, this timer makes us who we are: diurnal animals deeply attuned to a most basic feature of the environment. If we throw off that synchrony repeatedly, we are at risk of developing disorders from sleep disturbances to cancer. Yet a surprising force stronger than dusk and dawn can easily break its flow, scientists say, pushing our bodies into a precarious abyss of biological malfunction. It's not pollution. It's not heavy metals. It's not even stress.
While the master clock governs our sleep and wake schedule, it doesn't coordinate how the liver, stomach, and kidneys work in tandem to perform their functions. It is food that kick—starts the digestive machine when we first eat in the morning, and it is food that has the power to restart it at midnight, overriding the brain's directive to sleep. Food is imperative for survival, and it has not always filled our pantries; as a result, the body responds to it automatically, day or night.
Long commutes, demanding careers, round—the—clock TV, and the ever—on Internet keep us awake much longer than our ancestors ever imagined. Our social life lasts way past sunset—totaling 14 to 16 hours, or more—just when our body clock should be winding down.
As long as we are awake, we tend to troll for food. "We may be spending up to 18 hours a day eating," observes biologist Satchin Panda of the Salk Institute. We may not be chewing every waking moment, but we are munching enough to deprive the body of the time it needs to burn the fuel it has consumed.
To successfully process what we take in, we have to fast. "When we fast, our body turns on hundreds of thousands of genes that break down fat and cholesterol," Panda reports. Yet, if we eat continuously, they never engage. Just to turn on the natural metabolic furnace, we must fast for five or six hours. And then we must continue fasting for about the same amount of time to burn the calories accumulated during our awake time. The early birds have it right.
"Major fat burning occurs when we sleep," says neuronutrition researcher Oren Froy of Hebrew University in Jerusalem. During the nighttime fast, the liver switches from burning carbohydrates to burning fats, breaking lipids into glycerol and fatty acids. In short, the process makes us leaner and healthier. The longer the fast, the more calories burned. People who sleep more weigh less.
Although fasting is key to a healthy metabolism, supplying our digestive conveyor with the right foodstuff at the right time is important as well. Metabolic hormones, such as adrenaline and cortisone, peak in the morning; anything consumed between dawn and noon is immediately transformed into vigor and strength.
"That's why you must eat in the morning," says Froy's colleague Daniela Jakubowicz, even though traffic jams, school start times, and early meetings encourage us to skip breakfast. Scientists are nearly unanimous that it is the most important meal of the day; it synchronizes our brain timer with our food—processing machinery, setting the entire body to run in metabolic harmony for the day so that we function at our best.
A hearty breakfast that supplies both proteins and carbohydrates is ideal. If you don't eat protein in the morning, Jakubowicz warns, your system robs it from your muscles, which ultimately decreases metabolic capacity. Protein also boosts the ability to focus, while consuming carbohydrates reduces levels of ghrelin, the hunger hormone.
At breakfast we can safely indulge—donuts, chocolate, and cake—secure in the knowledge that they will be actively metabolized in a high calorie-burning phase. Plus, Jakubowicz says, those early carbs will help curb afternoon sugar cravings. For lunch, anything goes—fats, proteins, carbs—as long as it's consumed around noon, while our metabolic hormones are still at high tilt.
In the second part of the day, adrenaline and cortisone levels drop, and metabolism slows. If we binge on chips and chocolate bars in the afternoon or late at night, those calories stick.
That's why dinner is best eaten light and early, from 5 to 7 p.m. Froy suggests limiting protein at dinner because it stimulates concentration and can interfere with sleeping. He also recommends skipping the fats but keeping the carbs.
About six hours after we consume carbohydrates, our body secretes the hormone leptin, which signals satiety to the brain. "It's good to have carbs during dinner," Froy notes, because they keep hunger at bay during the night.
We tend not to think much of a little midnight snack, but for the human body even a small candy constitutes an Eating Event. "Multiple organs have to work together in a very precise manner to digest a bite of cookie," Panda explains. Eating before bedtime doesn't bode well with the clockwork metabolism: The stomach doesn't absorb nutrients efficiently when we sleep, leptin secretion is poor, and we feel hungry even if we have eaten enough.
But more important, late—night snacks break our fast too soon. Our metabolic genes never get to turn on to burn through the nourishment we have accumulated during the day.
The 10— to 12—hour fasting window is important for healthy and robust metabolic function, and shrinking it can lead to such disorders as diabetes and obesity.
It may sound counterintuitive, but if we supply our digestive conveyor with the right food at the right time while shutting it down at night, we can eliminate cravings, enjoy desserts, and feel full despite staying lean. Our calories will literally melt in our dreams.
Two researchers on opposite sides of the world conducted similar feeding experiments and came to similar conclusions. Oren Froy in Israel and Satchin Panda in California both found that mice that never fasted grew more obese than mice having daily fasting periods—although both groups consumed the same amount of calories. The mice that alternated eating and fasting were not only leaner but healthier, even when fed fatty food. They were even fitter than mice downing a low—fat diet.
Panda put mice consuming a variety of diets into a rotating drum and slowly increased the speed. "Usually mice eating high—fat food ad libitum are first to fall off. But the rodents whose high—fat regimen alternated with regular fasting periods held on the longest," says Panda. "We tried the drum experiment many times and they were always the best athletes!"