Can humans shorten their sleeping patterns? Can sleeping patterns be trained, or are they genetic? I read that a human needs to sleep at least 4 hours. Can sleeping time be reduced without having a negative impact?

Polyphasic sleep can allow people to sleep at multiple regular intervals throughout the day and allows people to get by with less total sleep. I don't know of any training that can help people reduce the amount of monophasic sleep they need without a negative impact.