How much sleep do we need? We are all different. Some people need only three hours of sleep a night. Others need ten hours of sleep a night. Most Americans sleep an average   of seven to eight hours a night. After the age of fifty the average sleep time goes down to 6.5 hours a night. Most people have a bad night when they can not sleep. About one in three Americans has a problem with sleep. Many of these people can not fall sleep. This problem is called   insomnia. This is not a new problem. Many famous people in history had insomnia. Some of these people had special   ideas to another to make them sleep. Benjaming Franklin, the famous stateman   and inventor  ,had four beds. He moved from one to another to fall asleep. King Lousis XIV of France had 13 beds and hoped that he could fall asleep in one of them. Mark Twain, the famous American writer, had a different way. He lay on his side on the end of the bed! The problem is called insomnia. Here "insomnia" means  _  .
going to sleep easily sleep at daytime the ways of sleep not being able to go to sleep