Skimping on sleep weakens your immune system, makes you more of a pessimist and increases your risk of gaining weight—among other recent findings. And yet more folks than ever before are dealing with the increasingly faster pace of life by staying up later and/or getting up earlier. More than a third of adults in the U.S. get less than seven hours of sleep each night, which may be one reason why drowsy driving, based on 2013 statistics, takes the lives of around 800 Americans and injures about 44,000 annually.

Considering that humans spend about a third (or less) of their lives in an unconscious state, scientists know surprisingly little about sleep's main function in our lives—other than preventing drowsy driving. Yet "research conducted over the past 20 years has finally begun to provide at least a partial explanation for why we must sleep," Robert Stickgold writes in the October 2015 Scientific American. In the article, Stickgold, director of the Center for Sleep and Cognition at Beth Israel Deaconess Medical Center in Boston, takes a closer look at the latest studies on sleep's role in strengthening memory (including the finding that different types of memories are enhanced during so-called REM versus non-REM sleep), mood, weight gain and immunity.

Stickgold also talks about the interplay between sleep, memory and dreams in the following video.