8 a.m. or 10 p.m.:

If you're going over notes for today's presentation or memorizing the names of your child's classmates' parents before the school open house tonight, do it early in the morning, when your immediate recall is highest. For longer retention (the book club meets in three weeks, but this weekend's your only chance to finish The Good Nanny), evening is better. "This is just the way the brain is organized," says Michael Smolensky, a professor of environmental physiology at the University of Texas School of Public Health at Houston and author of The Body Clock Guide to Better Health (Owl Books, $11, www.amazon.com). "Memory depends on nucleic acids, and those show circadian rhythms." In other words, your brain doesn't store information with the same efficiency all day; there are peaks and valleys. "College students often unknowingly take advantage of the dual circadian rhythm by staying up late studying, then doing a quick review the morning of the exam," says Smolensky.