Welcome to Your Brain (7 page)

and food are likely to be available. The circadian rhythm can run on its own, on an approximately

twenty-four-hour cycle, and can be reset by correctly timed light exposure. It’s synchronized with the

daily cycle of light and darkness, which is detected by your eyes. Your circadian rhythms regulate a

host of activities, including when you need to sleep, your body temperature, and when you get hungry.

However, these days, circadian rhythms can also trip you up. Nearly anyone who has traveled

long distances by airplane has experienced the problem of jet lag. For instance, we wrote part of this

book at a study center in Italy. We loved the beautiful setting and the opportunity to get away from our

day jobs and concentrate on writing, but there was one catch early in the trip: we found ourselves

writing at three in the morning. At breakfast, we engaged in fascinating conversations with other

residents, yet sometimes we could barely keep our eyes open.

Jet lag is a product of modern transportation: horseback, dogsled, and even car travel are slow

enough that the circadian rhythm can adjust to keep itself in sync with local time. Indeed, the first

report of jet lag came in 1931, when two pioneering aviators, Wiley Post and Harold Gatty, flew

around the world in a little less than nine days. They experienced the symptoms we recognize today:

difficulty getting to sleep, drowsiness, lack of alertness, and digestive problems.

Jet lag happens when your circadian rhythm has different timing from the external day-night cycle

in the world. As a result, your brain wants to sleep when it should be awake and vice versa. The

brain has a master clock, which normally sets the rhythms for body temperature, hunger, and sleep.

With jet lag, these rhythms can get out of sync with each other, causing symptoms like being hungry in

the middle of the night.

How light drives circadian rhythms can be explained by an analogy to a child on a swing. The

child and swing have a natural period over which one cycle of swinging tends to occur, but if you

push the swing, it will change speed. Push when the swing is going forward to make it go sooner;

push when it is coming back, and it goes later. In this way, you can alter the starting time of your daily

cycle, by exposing yourself to light. To influence your circadian rhythms, though, you must be in the

light at the right time of day.

Practical tip: Frequent jet lag and brain damage

Jet lag is not simply annoying; in repeated doses, it can be dangerous to your brain’s

health. People who frequently cross many time zones can experience brain damage and

memory problems. In one study, flight attendants with five years of service who repeatedly

took less than five days between long trips were compared to flight attendants who had two

weeks or more between trips. (That’s still a lot of flying!) Both groups flew the same

number of miles overall. The short-interval group had less volume in the temporal lobe—a

part of the brain involved in learning and memory. This group also had problems on a

memory test, suggesting that frequent travel had damaged their brains.

The brain damage probably resulted from stress hormones, which are released during

jet lag and are known to damage the temporal lobe and memory. Luckily, unless you work

for an airline, you probably don’t need to worry about this problem, since very few people

fly across multiple time zones more often than every two weeks. More likely to be at risk

are people who do shift work. Like repeated jet travel, frequent drastic changes in working

hours are likely to cause stress on the body and brain.

Light acts on circadian rhythms by driving cycles of activity in a tiny region at the bottom of your

brain called the suprachiasmatic nucleus, which acts as the master clock. The suprachiasmatic

nucleus receives signals from the eye and also generates its own rhythm. Indeed, cells from the

suprachiasmatic nucleus grown in a culture dish generate patterns of increasing and decreasing

activity on an approximately twenty-four-hour cycle. These cells are necessary for normal circadian

rhythms; animals with damage to the suprachiasmatic nucleus wake and sleep at odd hours.

Speculation: Morning people and night people

A tendency to function better at very early or very late hours might result from having a

natural circadian cycle that is not exactly twenty-four hours long. A twenty-three-hour

period would encourage early rising in people whose bodies are impatient for the day to

begin, while the twenty-five-hour person is still whacking away at the snooze alarm.

People with long circadian periods might also make different adjustments to jet lag. On

average, more people report difficulties when forced to rise earlier (as in eastward travel)

than when forced to rise later (as in westward travel). Difficulties with eastward travel

might be associated with periods longer than twenty-four hours. If this is the case, then

morning people might have more trouble with westward travel, and night people more

trouble with eastward travel—and both these traits would correlate with the natural cycle

of a person’s clock.

You can help us test these ideas by taking a quiz to see how you score. Report your

answers on our Web site at http://welcometoyourbrain.com, and see the results from others.

Quiz

1. When during the day are you most alert? (a) morning or (b) evening or night.

2. In the first two days after a long-distance flight, is it harder for you to adjust to (a)

westward travel or (b) eastward travel?

Scoring your answers

Our hypothesis predicts that most people would fall into one of these two categories:

Natural circadian period less than twenty-four hours (morning person type): 1) a, 2) b.

Natural circadian period more than twenty-four hours (night person type): 1) b, 2) a.

Light also triggers the production of the hormone melatonin, which is made by the pineal gland, an

organ the size of a large pea that hangs at the bottom of your brain, near the hypothalamus. Melatonin

levels start rising in the evening, peak around the onset of sleep, and go down again in the early

morning before you wake up.

Incidentally, the pineal gland has quite a romantic history. Several hundred years ago, the

philosopher René Descartes thought the pineal gland was the source of consciousness because there

was only one of it, and there is only one of you. That was wrong. It just goes to show that even the

smartest people can make mistakes when they construct arguments out of thin air.

Most people have a circadian period that is not exactly twenty-four hours, but we don’t usually

notice because the sun helps keep us on time. When people are left in a room with no lighting cues,

they inevitably drift across the clock, and eventually wake, eat, and sleep at hours out of

synchronization with the rest of the world.

Blind people, who do not have a way for light information to get from the eye to the brain,

naturally experience this sort of circadian drift. As a result, the blind often have disrupted sleeping

patterns. This shows that physical activity and social cues are not enough to keep people’s rhythms in

sync. The same is true of blind fish that live in caves; these critters don’t seem to ever sleep. The

dependence of daily habits on light is indeed universal.

Chapter 5

Bring Your Swimsuit: Weight Regulation

The sad truth is that your brain isn’t going to help you if you get fat. From an evolutionary

perspective, fat is much better than the alternative, starving to death. Of course, if your brain were

smarter, it would take into account that food is abundant in the modern world and that obesity is

responsible for three hundred thousand deaths per year in the U.S. But our brains aren’t built that way,

so we simply have to learn to live with weight regulation systems that developed around the need to

store food.

Because weight regulation is so important, multiple overlapping systems work toward keeping

your weight at the level that your brain considers appropriate, which is sometimes called your “set

point.” For example, scientists know of more than a dozen neurotransmitters that tell the body to

increase weight, and more than a dozen that tell the body to decrease weight. When you try to change

your weight by eating less, your brain falls back on tricks to keep your weight at its preferred level.

One is to decrease your resting metabolic rate, which is the amount of energy that you use when sitting

still. Another is to make you hungry, so that you’ll want to eat more. Finally, your brain may try to

fool you in the ways that we discussed in
Chapter 1.
When you find yourself acting as if cake doesn’t

have as many calories if you eat it in tiny bites from someone else’s plate, you’re falling for your

brain’s lies.

Your brain uses several indicators to keep track of your body’s energy needs. A hormone called

leptin is produced by fat cells and released into the blood. Leptin tells the brain not only how much

fat is present in the body but also how fat levels are changing. When your body fat decreases, leptin

levels in the blood fall sharply, telling your brain that the body needs more energy. These declining

leptin levels trigger hunger and weight gain. In contrast, when leptin levels increase, animals reduce

their food intake and lose weight, and people report being less hungry. Leptin receptors in the brain

are found in the arcuate nucleus of the hypothalamus, a part of the brain that is an important regulator

of many basic systems, including body temperature and sexual behavior. Leptin also acts at other sites

in the brain and elsewhere in the body, influencing metabolism and other regulators of fat storage.

My doctor told me to stop having intimate dinners for four—unless there are three other

people.

—Orson Welles

Insulin is another important signal that tells your brain how much stored body fat is available.

Produced by the pancreas after meals, it is released into the blood to tell a variety of cells to take up

glucose from the blood and store the energy. On average, lean animals have lower levels of

circulating insulin than fat animals, though insulin varies much more over the course of each day than

leptin. Leptin is a good measure of subcutaneous fat, while insulin is related to the amount of visceral

fat, which is a more significant risk factor for diabetes, hypertension, cardiovascular disease, and

many cancers.

The brain doesn’t like to take fat out of storage for everyday energy needs, saving it instead for

emergencies. It’s a long-term strategy, just as it’s better not to dip into your retirement account to buy

gas for your car. Thus, neurons in the hypothalamus and the brainstem also monitor available energy

sources to control food intake. For example, fatty acids and a hormone called peptide YY seem to act

directly on neurons to reduce eating, while the hormone ghrelin is released around mealtimes to

increase hunger and eating. These regulatory systems, probably along with others that are yet to be

identified, interact to determine whether your brain detects an energy deficit or a surplus at any given

time.

Did you know? Calorie restriction and life extension

In the 1930s, scientists found that rodents kept on a low-calorie diet lived about 50

percent longer than their freely fed counterparts. To varying degrees, the same effect has