What one bad night does to you

We all know the feeling. You go to bed late, your alarm rings at six, and you get out of bed on the wrong side. For the rest of the day you feel slow and grumpy. Most people think of sleep as lost time, something they can cut when life gets busy. Scientists see it very differently.

While you sleep, your brain is busy. It sorts through the day's information and decides what to keep and what to throw away. For example, if you practise a new song on the guitar in the evening, you often play it better the next morning, because your brain has repeated it during the night. Sleep also helps your body repair itself and fight illness.

So what happens when we don't sleep enough? After just one short night, most people find it harder to concentrate and remember things. They make more mistakes and lose their temper more easily. In one well-known study, people who had stayed awake for about nineteen hours did as badly on some tests as people who had drunk a glass or two of alcohol.

Adults need between seven and nine hours a night, and teenagers need even more. Yet many of us sleep far less, often because of screens. The light from phones tells the brain that it is still daytime, so it becomes harder to fall asleep. Worse still, one more message or one more video can keep us awake long after we planned to stop.

The good news is that small changes help. Try to go to bed and get up at the same time every day, even at weekends. Leave your phone in another room an hour before bed. Instead of thinking of sleep as wasted time, think of it as an investment in tomorrow. Nobody can burn the candle at both ends forever.