The human brain needs "offline" time to recharge, which experts say explains why people need sleep after they have been up for several hours, or after they've absorbed too much new information. Research on rodents at the University of Wisconsin (UW) Madison shows their brain's nerve cells called synapses are strong when they are awake and weak when they are asleep.

Synapses consume about 80 percent of the human brain's energy and because millions of neurons have thousands of synapses, the energy needed "is huge and can't be sustained," explains Chiara Cirelli, associate professor at the UW Madison School of Public Health and Medicine.

The brain needs an "offline period, when we are not exposed to the environment, to take synapses down," Cirelli writes in Nature Neuroscience. "We believe that's why humans and all living organisms sleep. Without sleep, the brain reaches a saturation point that taxes its energy budget."

Contrary to some beliefs that synapses work hard during sleep, Cirelli believes "learning occurs only when we are awake, and sleep's main function is to keep our brains and all its synapses lean and efficient."