Some people wilt with less than eight hours of sleep, while others thrive energetically with only six hours. What gives?

While all humans need sleep to survive, the number of hours of sleep an individual needs to be healthy and well-rested varies widely, and sleep continues to be an mystery to researchers.

Scientists studying sleep in fruit flies and other animals have found differences in the genes involved in sleep that may explain the wide variation of successful sleep times.

A 2009 study identified a rare gene mutation in a mother and daughter who slept only six hours a night with no apparent health impact. When the gene mutation was introduced into a small population of mice, the mice, on average, were awake a little more than one hour more than those with no mutation.

More recently, scientists studying fruit flies selectively bred the flies to sleep for shorter and longer periods of time. After 13 generations of fruit flies, they had flies that slept for an average of only 3.3 hours in a 24-hour period on the short end, and flies who needed almost 10 hours more sleep than that on the long end. While this finding shows that genes determine how long an animal needs to sleep, the researchers also found the process to be extremely complex — they found at least 126 different changes in 80 different genes through time. You might want to sleep on that.

So how much sleep do you need? Scientists tout the importance of sleep quality as well as quantity. People need deep sleep to restore and regenerate bodily functions. Regardless of whether you feel rested with five hours or nine hours, be sure to get as much snooze time as you need to stay healthy.