I am not sure there is much science behind the 8 hour recommendation itself. I have started recently hearing about studies that actually have found closer to 6 or 7 hours to be "optimal"

One example is: Patel, Sanjay R;Ayas, Najib T. "A Prospective Study of sleep duration and mortality risk in women". Sleep. 2004, 27:3.

From the abstract:

These results confirm previous findings that mortality risk in women is lowest among those sleeping 6 to 7 hours. Further research is needed to understand the mechanisms by which short and long sleep times can affect health.

Additionally, Youngstedt, SD. "Long sleep and mortality: rationale for sleep restriction". Sleep Medicine Reviews. 2004. Vol 8:3. pp159-174.

They start with:

Epidemiologic studies have consistently shown that sleeping >8 h per night is associated with increased mortality. Indeed, the most recent American Cancer Society data of 1.1 million respondents showed that sleeping longer than 7.5 h was associated with approximately 5% of the total mortality of the sample

But then it seems you should not sleep less than 5 hours either...

Gangwisch, JE, Heymsfield, SB et al. "Short Sleep Duration as a Risk Factor for Hypertension". Hypertension. 2006;47:833.

The found:

Sleep durations of ≤5 hours per night were associated with a significantly increased risk of hypertension (hazard ratio, 2.10; 95% CI, 1.58 to 2.79) in subjects between the ages of 32 and 59 years

So it would seem that between 6 and 7 hours, and definitely not more than 8 is your "best bet" based on the current science.

Not quite on topic, but related.

Quirks and quarks recently did a brief story (less than 5 minutes) on sleep debt, that adds to the general subject of the question.

http://www.cbc.ca/quirks/media/2009-2010/mp3/qq-2009-11-14_06.mp3

In short, you can 'catch up on sleep' though sleep debt can accumulate and be carried over long periods of time, and that people tend ro carry a chronically long sleep debt.

They also discuss that the amount of sleep we need is seasonal, is circadian, and it is implied that the optimal length of sleep is variable from person to person.

6-7hrs works for me..

But I'm sceptical about the 8hrs-a-night actually being bad for you. I'd need convinced that people who sleep 8hrs+ aren's just lazy feckers and more likely to have bad lifestyles.

People do sleep less than they used to in the UK, but I'd stay in bed longer if I had no central heating, TV or internet.

It is the time (rather than duration) of sleeping that is crucial to health, I would suggest, it's vital to sleep in the dark for the body to produce melatonin. Although there seems to be a slight tendency in parts of the world towards a slowdown in winter, we are not especially seasonal creatures (compared to those that hibernate at least), and our patterns remain much the same throughout the year. So, we have probably evolved to make sure of enough melatonin production time (along with other repairs and cycles that go on during sleep) when the days are long. Not many habitable places really have much less than 6 hours of darkness in summer, so logically that ought to be enough, although a little more in winter seems like it wouldn't do much harm either. Much less than 6 seems like it would be unhealthy.

Having said that, Margaret Thatcher claims to have needed only four, and she's still going strong at 186 years of age.