Seven—that's the average number of hours slept each night by women who scored highest on cognitive tests (reasoning, vocabulary, memory and more). When sleep decreased to six or fewer hours nightly
, scores plummet—especially in reasoning and vocabulary—and the brain effectively aged by four to seven years, found a five-year study published in the journal SLEEP
. While correlation isn't causation, sleep has restorative properties—and the habitual seven hours has also been linked with a lower dementia risk later in life.