It has long been known that mortality rates vary with sleep times. Epidemiological studies have found that long sleepers have higher death rates than regular sleepers, and short sleepers also have higher death rates. Scientists who study this question arbitrarily set the definition of short sleeping as either 5 or 6 hours per night, on a regular basis.
Researchers at Penn State University did a long-term study of people with "insomnia with objective short sleep duration" and found they were at higher risk for hypertension and diabetes and that the mortality risk in men, more than women, was greater in those who slept less than 6 hours per night.
Just considering men, the results of the Penn State study show that normal sleep times are correlated with the lowest mortality risk. In order of increasing risk:
1) normal sleep duration and no insomnia
2) normal sleep duration with insomnia
3) short sleep duration with no insomnia
4) short sleep duration with insomnia
In their statistical analysis, the Penn State researchers tried to separate out the effects of hypertension and diabetes and found that people in the fourth category, short sleep duration with insomnia, still had a significantly higher mortality risk. There must be something else going on besides hypertension and diabetes.
Oddly, the Penn State researchers found a minor, but statistically insignificant increase in mortality in women who were short sleepers. Why there is a difference between the sexes is not clear.
Further, the researchers concluded that after they adjusted for age, apnea, depression, obesity, alcohol use, smoking, and depression, there was still a correlation between short sleeping and early death. The authors concluded that for short sleepers, the increased risk of death from insomnia was comparable to the increased risk from sleep-disordered breathing.
Other researchers found that people with primary insomnia tend to have high levels of interleukin 6 in their blood, which may indicate either a problem in the immune system or a reaction by the immune system.
Other studies have found a relationship between short sleep and insomnia with hypertension And presence of inflammation markers in the blood The immune system and sleep have a complex, but tight, relationship.
It has also been found that adults in the 30-45 age range are more likely to have metabolic syndrome (a combination of high blood pressure, cholesterol problems, and high body mass index) if they are short sleepers. - http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2398755/
Short sleep is also associated with reduced cognitive functions in middle-aged and older people. http://www.journalsleep.org/ViewAbstract.aspx?pid=28123
Young adults are much less likely to die from any cause than old people, but there are markers we can examine to see what the effects of short sleep are. And it has been found that short sleep in young adults is associated with lower general health (self-reported). Long sleep does not appear to be a danger in young adults.
Young adults are more likely to push themselves to sleep less than they need, so there is always more of a question with that age group than others. A small study found that short sleepers carry a greater sleep debt, suggesting these individuals may be imposing self sleep restriction.
Short sleepers are more likely to eat a carbohydrate-rich diet http://www.journalsleep.org/ViewAbstract.aspx?pid=27900
And the relationship between sleep and obesity is detailed here.
Longitudinal studies have found short sleepers are more likely to develop Type II diabetes. Short sleep in very young (pre-school) children is often an indicator of future obesity problems. Daytime napping does not appear to compensate for the reduced sleep at night.
In the 1980s the American Cancer Society sponsored Cancer Prevention Study II. It concluded "that short sleep and insomnia seem associated with little risk distinct from comorbidities" http://archpsyc.jamanetwork.com/article.aspx?articleid=206050 but other studies have found otherwise.
A meta-study published in the journal Sleep in 2010 found "there is no evidence that sleeping habitually between 6 and 8h per day in an adult is associated with harm and long term health consequences" and "However, sleeping 9 h or more per night may represent a useful diagnostic tool for detecting subclinical or undiagnosed co-morbidity. People reporting consistently sleeping 5 hours or less per night should be regarded as a higher risk group for all-cause mortality."
The full study is here: http://www.journalsleep.org/ViewAbstract.aspx?pid=27780
Intentionally sleeping long nights for a limited period could be useful if it brings out subclinical or subtle co-morbidities, provided those co-morbidities can be addresses. We know of no doctors who systematically use this process for diagnosis, though.
Shift-work sleep disorder is a recognized condition that affects people who work odd hours and can result in diminished sleep times. Workaholism is known to negatively affect sleep time and quality. This brings up the question of which job categories are most associated with short sleep. Analysis of available data found managers were most likely to have short sleep, followed by workers in the transportation/warehousing and manufacturing industries. Is this because these jobs attract people who tend to sleep less or because of the effect of the job on the individual? Probably both. High mental-energy people may be more likely to be hired into managerial positions, and are also more likely to sleep less. Transportation workers includes truck drivers, which is somewhat worrisome given the dangers of drowsy driving.
Not much. And a tendency to not require a lot of sleep does not necessarily put you at risk. If you have chronic insomnia, work with a doctor to address that problem, as alleviating insomnia will raise your quality of life in the short run and probably make you less susceptible to other illnesses in the long run. But if you are one of those people who feels fine on less than 5 hours a night, go about your life and don’t worry about it.
Indeed, natural short sleepers are often envied by the rest of the population. The Wall St. Journal printed a story about the 1% to 3% of the population able to get by with little sleep – the "sleepless elite" they were called.
Be careful, though, because doctors know that many more people think they can operate at top performance on 5 hours/night than actually can. Know thyself – a good prescription for much in life including sleep behavior.