Chronic Inadequate Sleep.

The young men in the same study also had reduced levels of the stress hormone cortisol, which normally surges just before waking from a good night's sleep, energising people for the day's demands. The study participants had the low morning levels of cortisol, typical of their grandparents.

And these volunteers also showed that, with chronic inadequate sleep, young people might be accelerating the beer-belly, pear-bottom problems typically linked to middle age. They were producing lower levels of growth hormone after less than a week of four hours of sleep. Growth hormone is largely secreted during the night's first round of deep sleep. As adults age, they naturally spend less time in deep sleep, getting less of the hormone that, in addition to driving childhood growth, plays a role in controlling the body's proportions of fat and muscle.

The University of Chicago study's findings were the first solid evidence that chronic partial sleep deprivation could have physical health consequences. Since then, researchers have begun to look deeper at the links between sleep and illness. A study published in the December 7, 2004 Annals of Internal Medicine found that when 12 healthy, young men were restricted to four hours of sleep for just two nights, levels of leptin, a hormone that signals satiety, dropped, while levels of ghrelin, a hormone that prompts appetite, increased.