Whatever the cause, the effect is the same. Each night many people sleep fewer than the eight hours that doctors recommend. The obvious consequences are well known. You can be drowsy during the day, think less clearly and can't find the right word so readily. Concentration spans drop, movements become less co-ordinated.
Children become irritable and angry and can have trouble at school.
But there is mounting evidence that the effects of sleep deprivation build up over time and can seriously harm your health. There is strong evidence that sleep deprivation damages your immune system, making you more liable to catch colds and flu. Original tests on sleeplessness focused on extreme deprivation for short periods, from which it is easy to recover. Only now are scientists looking at the long-term consequences of cheating sleep by just a bit, but every night. One study shows that if you have sleep apnoea, you are three times as likely to die of a stroke, and are also at increased risk of heart attack.