A May 23 supplement of the journal Nature examines how researchers are defining the various functions of sleep, from how we learn to the regulation of metabolism and immunity. The Outlook supplement of Nature, which is available online for full free access, also examines the modern problem of insufficient sleep and new ways being developed to treat troubled sleeping.  A commentary by AASM member Charles Czeisler, MD, PhD, explores how the use of electric lights at night disrupts sleep.