Researchers working out of Stockholm University’s Stress Research Institute in Sweden used data from a previous survey, which collected self-reported data from 38,015 people in 1997. They then cross-referenced this data with the national death register.
According to Bustle: "The researchers divided the people into several groups: Short sleepers, who tended to sleep for fewer than five hours per night; medium sleepers, who slept around seven hours per night; and long sleepers, who slept for nine hours or more per night."
The groups were then subdivided based on how their weekday sleep habits compared with their weekend habits.
The results showed that people who slept less than five hours every night consistently and people who slept at least nine hours every night of the week had increased mortality rates than those who slept fewer than five hours during the week but caught up on the weekends with seven to nine hours.