Our sleep time over the past century has been (36) by almost 20 percent.
Generally, adults need to sleep one hour for every two hours (37) , 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 (38) of how much sleep it is owed. My colleagues and I (39) the term sleep debt because (40) lost sleep is like a (41) 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 (42) to fall asleep during the daytime becomes stronger.
During a five-day (43) , if you get six hours of sleep each night instead of the eight you needed, (44) . Because sleep debt accumulates in an additive fashion, (45) From this perspective, sleeping until noon on Saturday is not getting enough to pay back the ten lost hours as well as meet your nightly requirement of eight; you would have to sleep until about 5 p.m. to balance the sleep ledger. (46) .