In the early 1900s, psychiatrist Dr. Wehr conducted an intriguing experiment into sleep. He had a group of people plunged into darkness for 14 hours a day with no way to tell the time. The idea was that without the disturbance of electrical lights, the natural sleep cycle would reveal itself. By week #4, each of the subjects slept four hours, awoke for two, then slept for another four hours. Each of them slept biphasically. Each of them slept polyphasically.  
This experiment, combined with historian Roger Ekirch's views on sleep cycles in the pre-industrial era, heavily suggest that monophasic isn't our natural sleep cycle. If this is the case, perhaps insomniacs don't have something wrong with them-- they are just more in tune with the natural cycle. Lying awake at night is a norm. With monophasic, we're sleeping terribly, terribly wrong.