A targeted study by researchers at the Kangbuk Samsung Hospital in Seoul, South Korea has found that getting too much or too little sleep can contribute to the "hardening" of arteries. This condition, caused by calcium buildup in the arteries, can be an early warning sign of heart disease or lead to heart attacks. The study involved over 47,000 men and women, with an average age of 42, who completed a sleep questionnaire and underwent a series of tests. These tests measured arterial stiffness and evaluated calcium and plaque deposits in arteries. The average duration of sleep among participants was 6.4 hours per night. Researchers categorized those who slept five hours or less each night as "short" sleepers, and those who slept nine hours or more each night as "long" sleepers. The study found that poor sleep quality can lead to stiffer arteries whether an individual sleeps too few or too many hours. Researchers subsequently determined that those who slept an average of seven hours per night and reported good sleep quality had the lowest levels of vascular disease.