All over the world, teenagers have a reputation for staying up late, then oversleeping the following day. These bedtime habits are not a matter of personal preference, however—they have biological origins. Recent studies have established that changes during puberty cause teenagers' body clocks to operate on a different cycle compared to those of older adults, shifting their sleeping and waking times forward by several hours. Although this shift is entirely natural, it can cause problems for adolescents. Forced to wake earlier than their bodies would like in order to attend school, many experience chronic sleep deprivation, which can cause low mood and affect their ability to concentrate and learn. Educational institutions experimenting with later start times to accommodate these findings have seen student performance dramatically increase.