> 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.

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're cooperating and will sleep as long as you can or they let you, and there doesn'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.