Q. How much sleep do I need each night, and does it matter what time I go to bed?

A. Sleep experts say most adults need between seven and nine hours of shut-eye each night for optimum performance, health, and safety. When you don’t get enough, you accumulate a sleep debt that can be difficult to “pay back” if it becomes too big. The resulting sleep deprivation has been linked to health problems such as obesity; decreased productivity; and safety issues in the home, on the job, and on the road. Regularly short-changing yourself on sleep can also impair spatial memory and deplete your body of hormones such as melatonin, DHEA, and growth hormone.

Women who don’t get enough sleep may also run a greater risk for heart disease, according to a long-term study by Brigham and Women’s Hospital in Boston. Researchers asked more than 71,000 healthy women about their sleep habits. A decade later the group had recorded a total of 934 heart attacks, including 271 fatal attacks. At the end of the study, the researchers found that the women who reported sleeping five hours or less per night were 45 percent more likely to develop heart disease than women who slept eight hours per night.

What time you go to bed can also affect your health. Although an occasional late night won’t cause too much harm, if you regularly stay up till the wee hours or work the nightshift, you may be disrupting your body’s natural circadian rhythm—the cyclical changes your body goes through every 24 hours. The long-term result, say researchers at Mid Sweden University in Sundsvall, Sweden, is a greater risk of developing peptic ulcers, heart disease, and psychological problems.

Regardless of what time you crawl between the sheets, it’s a good idea to try and get to bed at approximately the same time every night. Not only does this help balance your circadian cycle, it also helps stave off insomnia. If, on occasion, you just can’t drift off, try taking 400 mg of valerian (Valeriana officinalis). Calcium is another sleep inducer. According to Canadian researchers, the body eats up calcium as it goes through its sleep cycles. Taking 800 to 1,000 mg of calcium before turning in helps your body maintain adequate calcium levels—and gives you a better night’s sleep.

This Ask the Expert was written by Kim Erickson, an herbalist, health writer, and the author of Drop Dead Gorgeous (Contemporary, 2002).