This week in science: Why your cat won't eat; a possible explanation for why some places feel haunted; a simulated end of the Universe; and much more! Scientists Reveal The Optimal Amount of Sleep to Lower Dementia Risk [caption id="attachment_199209" align="alignnone" width="642"] (Mladen Zivkovic/iStock/Getty Images)[/caption] A major review study has pinpointed the optimal amount of sleep to reduce your dementia risk: seven to eight hours per night.