Sleep - we spend roughly a third of our lives doing it, and most of us don't give much thought to those hours we spend snoozing.
But millions of people who don't sleep well spend far too much time thinking about why they're still awake. Researchers are slowly learning more about sleep - and how a good night's sleep is important to every day functioning.
They're also finding that people who have difficulty sleeping have a variety of health problems. Those range from increased rates of obesity to increased blood pressure.
Psychiatrist Edward Suarez is fascinated by sleep. His research at Duke University in North Carolina has focused on it for the past decade. He says poor sleep can take many forms.
"It's a very complex notion," Suarez says.
"It involves people reporting difficult times falling asleep, other people fall asleep quickly, but they tend to wake up frequently during the night and can't get back to sleep," he says. "Some people indicate that their sleep is not restful, or disturbed, and others just report that they are sleepy in the daytime, indicating that their quality of sleep that night is not very good."
Suarez asked several hundred people about their sleep. He had them fill out questionnaires and took blood samples from them.
He found that the subjects who complained of poor sleep had more health risks, especially the women. The risks were worst in women who complained of having trouble falling asleep at night. Their blood samples showed high levels of proteins that mark an increased risk of heart disease and diabetes.
Suarez says trouble falling asleep was also associated with psychological distress: greater depression, greater hostility, greater anger. But, he says, these associations were only noted in women, not in men.
Suarez says he was surprised that poor sleep led to so many health effects.
"And it appears that poor sleep was associated with this mosaic of different factors that alone, incur a greater risk of heart disease and type 2 diabetes," Suarez says.
"Together (they) really identify a population that's highly likely to develop heart disease and type 2 diabetes down the road, because of the increased risk of showing more than one risk factor. And most of these were women."
In previous studies, Suarez has found other problems for women who sleep poorly. He postulates that there might be so much stress associated with being a woman - and especially a working mother - that it's affecting sleep in a profound way.
Suarez' research is published in the journal Brain, Behavior and Immunity.