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.
