In the last decade, research has shown that insufficient sleep makes it much easier to gain unwanted weight.

First, a University of Chicago study revealed that sleep-deprived adults produced more of the hunger-producing hormone ghrelin and less of the satiety-signaling hormone leptin. Later, a University of Michigan study discovered that every additional hour third graders spent sleeping reduced their risk of becoming obese sixth graders by 40 percent.

Follow-up research done at the University of Chicago determined that an increase in ghrelin and a decrease in leptin which makes gaining weight far more likely occurs when subjects lose as little as 60 minutes of sleep a night for three nights.

Furthermore, a study funded by the National Institutes of Health, Minnesota Obesity Center, and the Mayo Clinic last year found subjects who slept 80 minutes less for eight nights ate an average of 549 extra calories each day than normal. Compounding matters and contrary to what you would expect being awake the extra time did not increase daily caloric burn significantly.

In fact, a Swedish study that disrupted adults' sleep found this disruption caused the body to hoard calories and lower basal metabolic rate to such an extent that subjects burned 20 percent fewer calories the next day. For an average-sized male adult, the decrease would be more than 500 calories.

So what's clearly been established in the last 10 years is that those who sleep less than they'd like are far more likely to weigh more than they'd like.

But the fact that Americans are staying up longer than ever could be leading to other problems besides weight gain.

A study done at Johns Hopkins University and published in the November issue of Nature linked exposure to bright light at night to depression and learning problems. Samer Hattar, a biology professor at Johns Hopkins explained it in an Medical News Today article this way: "Chronic exposure to bright light, even the kind you experience in your own living room or in the workplace at night . . . elevates levels of a certain stress hormone [cortisol] in your body."

Cortisol is the hormone you secrete in massive amounts when you're half asleep and think you hear a burglar. It causes your heart rate and blood pressure to escalate, your breathing to quicken, and your liver to release instant energy.

While it's a great aid for supreme physical effort, the Johns Hopkins research on mice (mice are used because their eyes react to light in the same manner as a human's) found that minute levels of cortisol produced what was called "depression-like behaviors." The mice, for instance, no longer showed an interest in sugar or pleasurable activities.

After administering Prozac, however, a commonly prescribed anti-depressant, the mice again showed interest in both.

Additionally, when the mice were tested after erratic exposure to bright light, they did not learn or remember as well.

Before you dismiss the Johns Hopkins research simply because it was conducted on mice, consider humans and the converse a lack of light. Because it begins in late fall, peaks in the winter, disappears by summer, and is far more prevalent in Vermont than Florida, a type of depression called SAD, seasonal affective disorder, is believed to be caused by a lack of sunlight.

Doctors make the diagnosis of SAD instead of general depression when a patient's depressive symptoms wax and wane with the seasons for at least two years.

If a lack of sunlight adversely affects health, doesn't it stand to reason that an excess of artificial light can do the same?

And while exposure to bright light late at night might not be the only reason, it's been well established that depression is on the rise. The World Health Organization (WHO) predicts that by the year 2020, depression will be the second leading cause of death after heart disease.

WHO also estimates that 15 percent of the inhabitants in developed countries suffer from "severe" depression. A National Institutes of Medical Health fact sheet has the U.S. percentage at 10 percent.

Most frightening, however, is a Harvard University study that found a 23-percent increase in depression in children. That, along with the highly quoted stat that 15 percent of those who become seriously depressed commit suicide, should be enough to make parents think long and hard about allowing electronic gadgets in bedrooms at night, for if the gadgets are available, exposure to late-night light increases and sleep surely declines.