Though most people require seven to nine hours of sleep a night, scientists say a rare genetic mutation allows some people to feel alert and rested after getting just six. The mutation of the DEC2 gene, which is involved in the regulation of circadian rhythms(24時間リズム、生物リズム), is believed to be quite rare; researchers located just two carriers, a mother and daughter, out of more than 250 people. When the researchers bred mice to carry the same mutation, they found that those with the mutation slept less and needed less time to recover from sleep deprivation.