The Science of Sleep Debt

Many people treat sleep as a flexible resource, something to be cut short whenever life gets busy and made up for later. Scientists who study sleep, however, warn that the body quietly keeps a kind of account. When we regularly sleep less than we truly need, the missing hours accumulate into what researchers call "sleep debt." Like a financial debt, it does not simply vanish when we ignore it, and the longer it （ 1 ）, the harder it becomes to repay.

The effects of this debt are surprisingly easy to underestimate. Studies have shown that people who sleep too little for several nights in a row perform worse on tests of memory, attention, and reaction time, yet most of them firmly believe that they are functioning perfectly normally. In other words, tired people are frequently poor judges of just how tired they actually are. Sleeping longer at the weekend can help to some degree, but research suggests that a few extra hours （ 2 ） undo the damage caused by a whole week of poor sleep.

These findings have uncomfortable implications for modern societies, where long working hours and bright screens late at night have become normal. Some doctors now argue that chronic lack of sleep contributes to serious health problems, including heart disease, weakened immunity, and depression. （ 3 ） treating sleep as a luxury, or even as a sign of laziness, they suggest, we should regard it as a basic biological necessity, as important to our health as good food and regular exercise.