Researchers have long known that sleep is important for forming and retaining memories, but how this process works remains a mystery. A study published in March suggests that strange electrical activity, involving neurons that fire backward, plays a role.

Neuronal activity typically requires sensory input—for example, a taste or smell—that gets received by neurons' dendrites and then transmitted as an electrochemical message to other cells via long axons. Yet the brain is mostly closed off to sensory input during sleep. Instead evidence suggests that during sleep, neurons are controlled by electrical impulses that ripple through the brain like waves. In 2011 researchers found that these waves of electricity cause neurons in the hippocampus, the main brain area involved with memory, to fire backward during sleep, sending an electrical signal from their axons to their own dendrites rather than to other cells. The new work, published in the Proceedings of the National Academy of Sciences USA, confirmed this unusual behavior and suggested that firing in reverse weakens the dendrites' ability to receive input from other neurons.

Weakening neural connections may serve a dual purpose, says R. Douglas Fields, a laboratory chief at the National Institutes of Health and co-author of the study with neuroscientist Olena Bukalo and other colleagues. The authors suggest that firing backward helps to strengthen the electrical signals of neighboring cells, necessary to solidify memories, as well as freeing up space in the brain to store new memories on waking.

This study was conducted in samples taken from rat brains, but sleep is thought to induce backward firing in human neurons, too. In fact, Fields says, this bizarre electrical behavior may underlie the positive effects of deep-brain stimulation, which, though not well understood, has been shown to improve the symptoms of Parkinson's disease and other neurological disorders.