Individual differences in sleep duration are normal, but less than 6.5 hours of sleep may induce performance decrements and could be unhealthy.
Some people think they can do with a little amount of sleep and indeed short sleep may be in your genes. But in most cases, these short sleepers are just sleep deprived without actual noticing. Already 1.5h less-than-usual sleep during one night may induce a reduction of 32% in alertness. Chronic sleep restriction to 6h of sleep on 6 consecutive days, may result in 6 times more missed hits in an attention test. Epidemiological studies show that people who sleep less than 6 hours per night have an increased risk of developing or dying from a coronary heart disease (+48%) or stroke (+15%).