No one knows why we sleep, but it’s certain that we need to. People who are prevented (阻止) from sleeping begin to suffer obvious (明显的) effects after a few days—they think less clearly, and they fall asleep during the working hours.

There are no rules about sleep. Generally speaking, grown-ups sleep about 7 and a half hours each night and probably more than 60 percent get between seven and eight hours. But perhaps eight percent are quite happy with 5 hours or less, and four percent or so find that they want ten hours or more. If you feel all right, you’re probably getting enough sleep. The important thing is not to worry how much other people get—their needs may be different. Exercise doesn’t seem to increase the need for sleep—office workers, for example, sleep for about as long as people doing physically active work.

Children sleep more than grown-ups—perhaps 14 to 18 hours soon after birth, going down to grown-up levels by early teenage. Sleep patterns also tend (倾向) to be different in the old people, who may sleep less at night than they did when younger, find sleep getting more broken, and often make it a rule to sleep during the daytime.

Is there a way to quickly remember information just before taking an exam? Walking backwards might be a good way. Scientists from the University of Roehampton in the UK said this activity can help people improve their short-term memory.

Researchers asked 114 volunteers to watch a video. After watching the video, volunteers were divided into three groups. One group was told to walk ten meters forward. The second group walked ten meters backwards. The third group stood in one place. All three groups were then asked 20 questions about what they saw in the video.

The scientists found that the group of walking backwards got two more answers correct on average than the other two groups.

This suggests that the relationship between the concepts(概念) of time and space is important considering how our minds form memories. “Time is really expressed through space,” Aleksandar Aksentijevic, who led the study, told the Daily Mail when you walk backwards, you see things from a different way compared to walking forwards. This difference helps people recall things that happened in the past.

And walking backwards is not just good for our brains, it is also good for the rest of our body. Compared to walking forward, walking backwards is more challenging. This can improve our health. according to New Scientist. Walking backwards uses more energy in a short time and burns more calories. Besides, walking backwards puts less stress on our knees. It could be helpful for people who often have pain in their knees. Walking backwards also keeps our spines(脊柱) strong, which can help to reduce pain in the back. This might be why many old people like walking backwards.