From The New York Times:
Today animals sleep in many different ways: brown bats for 20 hours a day, for example, and giraffes for less than 2. Sleep was once considered unique to vertebrates, but in recent years scientists have found that invertebrates likes honeybees and crayfish sleep, as well. The most extensive work has been carried out on fruit flies. “They rest for 10 hours a night, and if you keep them awake longer, they need to sleep more,” said Dr. Giulio Tononi, a psychiatrist at the University of Wisconsin. Discovering sleep in vertebrates and invertebrates alike has led scientists to conclude that it emerged very early in animal evolution – perhaps 600 million years ago.
Scientists have offered a number of ideas about the primordial function of sleep. Dr. Tononi believes that it originally evolved as a way to allow neurons to recover from a hard day of learning. “When you’re awake you learn all the time, whether you know it or not,” he said. Learning strengthens some connections between neurons, known as synapses, and even forms new synapses. These synapses demand a lot of extra energy, though. “That means that at the end of the day, you have a brain that costs you more energy,” Dr. Tononi said. “That’s where sleep would kick in.”
More here.