النص
The Science of Sleep
We spend roughly a third of our lives asleep, yet for most of history sleep was a mystery. Today, scientists understand far more about what happens when we close our eyes, and why missing sleep affects almost everything we do.
Two systems work together to decide when we feel sleepy. The first is our internal body clock, known as the circadian rhythm. It runs on a cycle of roughly twenty-four hours and is set mainly by light. When light reaches our eyes in the morning, it tells the brain that the day has begun. In the evening, as it gets dark, the body releases a hormone called melatonin, which helps prepare us for sleep. Bright light from screens late at night can delay this process, which is one reason why scrolling in bed may keep us awake.
The second system is often called sleep pressure. While we are awake, a chemical called adenosine gradually builds up in the brain, and the more of it there is, the sleepier we feel. Sleep clears it away. Caffeine works by blocking the effect of adenosine, which is why coffee makes us feel more alert, at least until it wears off.
Sleep itself is not one single state. During the night, we move through several cycles, each lasting around ninety minutes. Some stages are deep sleep, when the body carries out repairs and memories from the day are strengthened. Others are known as REM sleep, the stage in which most vivid dreaming happens. A good night's rest includes several complete cycles.
Teenagers face a particular challenge. During adolescence, the body clock shifts later, so many teenagers genuinely don't feel tired until late at night. When school starts early, they are woken up in the middle of their biological night. This is why some experts have argued that later school start times would benefit students.
Health organisations generally recommend that adults get seven to nine hours of sleep a night, and teenagers a little more. Regularly missing sleep has been linked to poorer concentration, worse mood and a weaker immune system. In other words, sleep is not wasted time. It is when the brain and body do some of their most important work.