Almost everyone needs at least eight, and no you can’t train yourself to get by on less:
In what was the longest sleep-restriction study of its kind, Dinges and his lead author, Hans Van Dongen, assigned dozens of subjects to three different groups for their 2003 study: some slept four hours, others six hours and others, for the lucky control group, eight hours — for two weeks in the lab. […]
Not surprisingly, those who had eight hours of sleep hardly had any attention lapses and no cognitive declines over the 14 days of the study. What was interesting was that those in the four- and six-hour groups had P.V.T. results that declined steadily with almost each passing day. Though the four-hour subjects performed far worse, the six-hour group also consistently fell off-task. By the sixth day, 25 percent of the six-hour group was falling asleep at the computer. And at the end of the study, they were lapsing fives times as much as they did the first day.
There are a few people who get by on four or less hours per night, but they’re rare. There are a lot more people who think they need six or less, but actually need a full eight. This Wall Street Journal Article covers the “sleepless elite” and how rare they. Current research indicates that there’s no way to modify your sleep requirements – they’re genetic.