Especially in some of the older papers, the sleepers' gains might have been due to the passage of time or to a period of silence and darkness, without interference or distraction, rather than sleep itself. The older studies also used notoriously stressful sleep-deprivation techniques, which may have additionally confounded the results. All of this matters because we want to know why sleep—in and of itself—is beneficial (and what exactly we might be depriving ourselves of when we try to trick the brain with caffeine or Modafinil).