Jerome Siegel, a professor of psychiatry at UCLA argues that sleep has evolved to optimize the use of time. It keeps animals safe when the hunting, fishing, or scavenging is scarce or risky. In this view, differences in sleep patterns, including periods of insomnia, are not seen as problems but adaptations.
There is opposition to almost ever idea offered about sleep. Some believe it consolidates memories, others think sleep is needed to repair neural damage. Critics also point out that sleeping animals are less alert to predators than waking ones.
Siegel points out that sleepers are less vulnerable than they appear, being highly sensitive to certain sounds such as thumps, voices, or a babies whine. He also argues that the need for sleep is diminished in the most important waking hours - killer whales migrating for weeks on end, fully alert, with no sleep, is an example.
Siegel says that the elderly, who general sleep much less than young adults, may not be sleep deprived because of less sleep. They no longer have the need grow, and they have more of a need and ability to do things for themselves.
Siegel comes to the conclusion that people simply may not sleep well, or at certain hours because there is work to be done.