Passage2People often complained about not gettinga good night's sleep，but sleep patterns differ from person to person. Most adults require six to eight hours of sleep to function well，while others survive on only a few hours. Still，most people today think of sleep as one continuous period of downtime. This is not the way people used to sleep. According to researchers in earlier times，people divided sleep by first sleep a few hours，waking up，then going back to sleep.Before the 18th century，people had no gas or electricity in their homes. Fire，candles，or oil lamps were the common forms of lighting. This lack of artificial lighting in homes contributed to people's sleep patterns. It made sense for people to go to bed early. If you live in this time period，you might be a hard-working farmer，and you would come home，eat and quickly fall into bed exhausted. You would probably go to sleep at 9：00 or 10：00 P.M. In this first period of sleep-called first sleep-you would typically sleep until midnight or shortly afterwards.Halfway through the night during a period some call the watch，or watching period. When you came out of first sleep，you would stay in bed and relax quietly. You might talk with a bedfellow，meditate on the day's events or the meaning of a dream，or just let your mind wander. If you enjoyed writing or drawing，you might get out of bed to write a poem or story or draw a picture.Then you would start to feel sleepy, so would return to bed and fall asleep again for your second sleep. This period would continue until early morning when daylight arrived. Again, with no artificial lighting in homes, people naturally woke up early to take advantage of sunlight.Today, human may consider divided sleep a strange habit, but sleep researchers say that it is actually a more natural sleep pattern. Dr. Thomas Wehr of the U.S. National Institute of Mental Health has studied human sleep. He thinks that modern sleep problems occur because the orderly, natural way of sleep is breaking through the more recent continuous sleep pattern. Wehr and other scientists believe that artificial lighting has altered the way people sleep. In a research study, he asked 15 adults to rest and sleep in darkness for 14 hours(from 6:00 P.M. to 8:00 A.M.). At first, the subjects took a few hours to get to sleep, and then slept 11 hours a night. Then overtime, they switched to divided sleep. They fell asleep for about 3 or 5 hours in the evening, stay awake for an hour of two and then slept again for four hours till early morning.Unlike the people in the study, we modern humans generally do not practice divided sleep. However, many of us have the experience of waking up in the middle of the night. We usually consider this a sleeping problem, but perhaps we should look at it as natural behavior. Divided sleep may be the way we should all be sleeping. A first sleep followed by a relation period and a second period of sleep could help all of us to beat the stress of our fast-paced lives.What is the main purpose of paragraph 2～4?