Not getting enough sleep? Some extra Zzzs each night may improve more than just your daytime alertness. New research shows more sleep may also improve your ability to withstand pain.

In fact, sleepy volunteers who got about two hours more sleep per night for four nights showed improvements in a test measuring pain sensitivity. Participants who got more sleep were also a lot more alert during the daytime.

"If you are already sleeping eight hours a night, you probably don't need more sleep," says researcher Thomas Roth, PhD. He is director of the Sleep Disorders Center at Henry Ford Hospital in Detroit. But "if you spend six hours in bed a night, spend eight -- preferably nine," he says.

The National Sleep Foundation recommends adults get seven to nine hours of sleep each night.

"For people who don't get enough sleep, sleeping longer decreases pain sensitivity," he says. This likely holds for all types of pain, including chronic back pain and other painful disorders, Roth says.

The study included 18 healthy adults aged 21 to 35 who did not have any pain. Half spent 10 hours in bed for four nights, and the others kept to their usual nighttime bed schedules. People in the extended sleep group raked in close to two hours more sleep per night due to their new bedtime ritual, an average of 8.9 hours per night vs. 7.14 hours per night among those who kept their own schedule.

The researchers measured pain by how long participants could keep a finger held to a heat source. The amount increased by 25% in those in the extended sleep group after just four days. Previous research suggests this is comparable to taking a 60-mg dose twice a day of the painkiller codeine.

Exactly how more sleep can help improve pain is not fully understood. "We think that sleep loss and pain both increase levels of inflammatory markers, but getting more sleep may help decrease this inflammation," Roth says.

The next step is to look at people getting ready for surgery to see if treating any underlying sleep issues can affect their pain sensitivity and the amount of painkillers they require, Roth says.

The findings appear in December issue of Sleep.

"When they extended their sleep, participants were able to withstand a greater time before they withdrew their fingers form heat," says Harley Greenberg, MD. He is the medical director of the North Shore-Long Island Jewish Sleep Disorders Center in New Hyde Park, N.Y. But "it is a big jump to apply this to patients with chronic pain syndrome."

The study period was brief, and getting more sleep on a regular basis may have even more pronounced effects on pain, he says.

"A relatively short-lived increase in sleep time in healthy adults reduces pain sensitivity," says Roger B. Fillingim, PhD. He is the director of the University of Florida Pain Research and Intervention Center of Excellence in Gainesville. "This is among the first studies I have seen to show that a modest sleep improvement reduces pain sensitivity."

Sleep expert Michael Breus, PhD, says that sleep and pain are intimately connected. "When you are sleepy, you are cranky, moody, depressed, and anxious," he says. "Every injury or type of pain gets worse with less sleep."