[填空题]Our sleep time over the past century has been{{U}} (36) {{/U}}by almost 20 percent. Generally, adults need to sleep one hour for every two hours{{U}} (37) {{/U}}, which means that most need about eight hours of sleep a night. Of course, some people need more and some less. Children and teenagers need an average of about ten hours. The brain keeps an exact{{U}} (38) {{/U}}of how much sleep it is owed. My colleagues and I{{U}} (39) {{/U}}the term sleep debt because{{U}} (40) {{/U}}lost sleep is like a{{U}} (41) {{/U}}debt: it must be paid back. If you get an hour less than a full night’s sleep, you carry an hour of sleep debt into the next day—and your{{U}} (42) {{/U}}to fall asleep during the daytime becomes stronger. During a five-day{{U}} (43) {{/U}}, if you get six hours of sleep each night instead of the eight you needed,{{U}} (44) {{/U}}. Because sleep debt accumulates in an additive fashion,{{U}} (45) {{/U}}From this perspective, sleeping until noon