Messages to eat are all around us. | Photo Credit: Thomas Hawk, CC BY-NC
Fatter and fatter
Many scientists believe that societal factors, such as advertising, have combined to create an environment in which the temptations to eat have overwhelmed our body’s natural biological ability to control what and how much we consume. The result is that in the United States, two-thirds of adults, and more than one third of children and adolescents, are now overweight or obese. This trend is spreading to other countries all over the world. Even worse, diseases that are associated with excess body weight – such as diabetes, high blood pressure and heart problems – are also becoming more prevalent.

At the core of the problem is the fact that many of the foods we can’t seem to resist are unhealthy. Some of the most attractive and popular foods in our current environment contain high amounts of saturated fats – high levels are found in red meats and dairy products like ice cream and butter. This type of diet is consumed by so many people in the US and other western societies that it is often called the “western diet.” No wonder obesity has become such a problem.

Beyond bellies to brains
Over the past several years, many scientists have reported that consuming a western diet and gaining excess body weight may have harmful effects on the brains of both human and nonhuman animals. For example, some research suggests that middle-aged adults who are overweight and obese are at greater risk for developing Alzheimer’s disease and other types of late-life cognitive dementias compared to people of normal weight. The results of other studies suggest that even children as young as seven years of age may suffer certain types of memory impairments as a consequence of consuming too much of a western diet and accumulating too much body fat.

Much information about the nature of the effects of western diets on the brain comes from studies with rats and mice. Research in our lab and elsewhere has repeatedly shown that feeding rats a diet with levels of saturated fat and sugar much like those in the human western diet weakens the blood-brain barrier (BBB). The BBB is a system of cells and membranes that form tight junctions to prevent harmful agents that circulate in the bloodstream from entering the brain. Feeding rats a western-style diet weakens those tight junctions and thereby allows potentially harmful substances to pass into the brain.

Healthy tight junctions keep substances in the bloodstream from diffusing into the brain. | Photo Credit: Chrejsa, CC BY-NC-SA
To determine which areas of the brain are most vulnerable to the ill-effects of a leaky BBB, we infuse a small amount of dye into the bloodstream of a rat and measure areas of the brain where the dye accumulates. In overweight rats fed a western-style diet, the dye appears to collect preferentially in the hippocampus, a brain structure involved with important learning and memory functions. As an apparent response to the accumulation of such intruding substances, the hippocampus becomes inflamed and its electrochemical activity changes. Rats that suffer these consequences also show deficits in their ability to use certain types of information processed by the hippocampus.

A vicious cycle
Do these deficits have anything to do with our ability to resist eating high-fat and sugary foods? We think they do. One type of information that is processed by the hippocampus takes the form of internal physiological signals about one’s need for food. Rats and people who have sustained damage to their hippocampus appear to have difficulty using those internal signals to tell whether or not they’ve had enough to eat or drink. In the presence of powerful cues in the environment that entice you to eat, a reduced ability to use information from your body that tells you that you don’t need food can lead to overeating.

Photo Credit: Author provided
The result could be a vicious cycle in which eating a western diet produces hippocampal dysfunction which weakens the ability to use internal cues to counter eating elicited by cues in the environment. This could lead to progressively more eating of western diet based on progressively greater deterioration of hippocampal function. As the hippocampus becomes more and more impaired, the severity and scope of learning and memory deficits would also increase. The result could be not only obesity but also more serious cognitive decline.

A dark night is good for your health
Today most people do not get enough sleep. The Centers for Disease Control and Prevention (CDC) has called insufficient sleep an epidemic. While we are finally paying attention to the importance of sleep, the need for dark is still mostly ignored.

That’s right. Dark. Your body needs it too.

Being exposed to regular patterns of light and dark regulates our circadian rhythm. Disruption of this rhythm may increase the risk of developing some health conditions including obesity, diabetes and breast cancer

Light regulates our sleep and wake patterns
The physiological processes that control the daily cycle of sleep and wake, hunger, activity levels, body temperature, melatonin level in the blood, and many other physiological traits are called the endogenous circadian rhythm.

Sunrise. | Photo Credit: Mathilde AUDIAU, CC BY-NC-ND
On its own, the endogenous circadian rhythm is nearly, but not exactly, 24 hours. Our bodies rely on the Sun to reset this cycle and keep it at precisely 24 hours, the length of our days. The light – and the dark – are important signals for the cycle. This circadian rhythm has developed over three billion years as life evolved on Earth in the context of the Sun’s day/night cycle. It is built deeply into our genetic makeup.

During the night, in the dark, body temperature drops, metabolism slows, and the hormone melatonin rises dramatically. When the Sun comes up in the morning, melatonin has already started falling, and you wake up. This natural physiological transition into and out of night is of ancient origin, and melatonin is crucial for the process to proceed as it should.

If you were to put someone in a dark cave with no time cues at all, the cycle will last about 24 hours, but not exactly. Without time cues like those from the Sun, eventually that person would become out of sync with people outside. In fact many profoundly blind people, who cannot perceive light, must cope with this de-synchronization in their daily lives.

Dark is good. | Photo Credit: Sign image via www.shutterstock.com.
What does your body do in the dark?
Many things happen to our bodies during the dark. Levels of the hormone leptin, which helps control hunger, go up. High levels of leptin mean we do not feel hungry while low levels make us hungry.

Why does leptin go up in the dark? Since we evolved without artificial light at night, one theory holds that leptin goes up at night because it would be good to not be hungry during the night, rather than needing to forage in the dark and possibly get into trouble.

This fasting that should happen every night, and why we call the first meal in the morning “breakfast.” Experiments in human beings have shown that sleep disruption and turning on lights lowers leptin levels which makes people hungry in the middle of the night.

In the last decade or two it has become clear that the genes which control the endogenous circadian rhythm (the “clock genes”) also control a large part of our entire genome including genes for metabolism (how we process the food we eat), DNA damage response (how we are protected from toxic chemicals and radiation), and cell cycle regulation and hormone production (how our cells and tissues grow).

Light at night disrupts these processes. The changes that result from exposure to electric light at night have biological connections to disease and conditions that are common in the modern world today including obesity, diabetes, cancer and depression.

Blue light from tablets can make it hard to fall asleep. | Photo Credit: Boy with tablet via www.shutterstock.com
Blue light, red light, no light
Not all light is the same – some kinds of light make you more alert and more awake, and others have less of an effect.

Light from the Sun is strong in blue, short wavelength light, although it includes all other colors as well. That’s important in the morning when we need to be alert and awake. But when it comes in the evening or during the night, it fools the body into thinking it’s daytime. We now know that this bright blue light has the strongest effect on lowering melatonin during the night.

Your tablet, phone, computer or compact fluorescent lamp (CFL) all emit this kind of blue light. So using these devices in the evening can prevent that primordial physiological transition to night from occurring. This makes it harder to sleep and might also increase the longer term risk of ill-health.

Other kinds of light, like dimmer long wavelength yellow and red light, have very little effect on this transition. This is the kind of light from a campfire or a candle; even the old fashioned incandescent light bulb is dimmer and redder than the new CFL.

Only in the last 20 years have we acquired a basic biological understanding of how the eye’s retina tells the circadian system it is daytime. Now we know that blue, short wave-length light is captured by the newly discovered photopigment melanopsin in the retina, and that when blue light stops, we start our physiological transition to nighttime mode.

It’s hard to find dark, even at night. | Photo Credit: NASA Earth Observatory, CC BY
Electricity changed the way we sleep
Before electricity, people experienced bright, full-spectrum days of sunlight and dark nights. We slept in a different way than we do now. The dark lasted about twelve hours and during this time people slept for eight or nine hours in two separate bouts, and were awake, but in the dark, for another three or four hours.

Everything changed when electric lighting was invented in the latter part of the 19th century. Since then there has been an ever increasing assault on dark. Outdoor environments are relentlessly lit, and more and more people use computer tablets and smart phones at all hours, bathing their faces in bright blue light at times of day when they should be transitioning to nighttime physiology.

When people get away from the city and its artificial light to go camping, they often notice a marked improvement in their sleep. A recent study has verified this effect.

Today, most of us get too little light during the day and too much at night for our circadian rhythm to function at its best. It is the rare person who sleeps in a completely dark bedroom, and many people get very little sunlight because they work inside all day long.

What can you do for your circadian health? Get bright, blue light in the morning (preferably from the Sun), and use dim, longer wavelength light (more yellow and red like incandescent) in the evening. And sleep in the dark.

This will certainly improve sleep, and may reduce risk of later disease.