We just can't sleep. Usually a person is annoyed but returns to normal sleep patterns within a night or two. After all, studies have shown that people can tolerate some periods of total sleep deprivation without much loss of performance. But we still don't like it. We should note that the length of time we need to sleep varies a good deal from person to person.
Some people do very well on five hours a night and others require much more.
The problem of chronic insomnia is serious enough that there are over 100 sleep-disorders centers throughout the United States and many other programs that study the nature of sleep. Each one is characterized by very different brain wave patterns with most of the activity occurring in the right hemisphere of the brain. In the early stages Alpha waves predominate, then sleep spindles appear in the brain as wave patterns slow down. Next, brain waves slow down even more and the spindles disappear. Then Delta waves appear and you are deeply asleep.