&gt;  Instead of using self-reported data, he wanted objective data—that which is recorded using sleep-monitoring instruments or by observers as participants slept in a lab. Youngstedt’s and his colleagues’ systematic review, published in Sleep Medicine Reviews, took into consideration 168 studies with objective data conducted between 1960 and 2013—involving more than 6,000 participants (understandably a much smaller set than the self-reported surveys) across 15 countries. It too reveals that the total sleep time hasn’t changed much in that period.<p>Why would you consider this as any evidence about whether the general population sleeps more or less? When you participate in a study, you are not making your usual decisions; you&#x27;re cooperating and will sleep as long as you can or they let you, and there doesn&#x27;t seem like much reason to expect sleep lab technicians to have gotten more impatient over the years and start rushing in to wake up subjects at hour 7.