It looks like your mother was right: when you've got a cold, sleep may be the best medicine.

German researchers have discovered one way sleep improves the body's ability to fight off a cold. Sleep, it seems, strengthens the potency of certain immune cells by improving their chances of attaching to-and eventually destroying-cells infected with viruses.

The researchers focused their attention on T cells, which battle infections. When T cells spot a virally infected cell, they activate a sticky protein known as an integrin that allows them to adhere to that cell. The researchers were able to prove that lack of sleep, as well as sustained periods of stress, lead to higher levels of hormones that appear to block the master switch that activates the sticky proteins.

If you want to have your immune system tuned up to fight off invaders, "get the needed amount of sleep every night and avoid chronic stress," said study leader Stoyan Dimitrov, a researcher at the University of Tubingen, Germany.

Dimitrov and colleagues suspected that certain hormones (such as epinephrine, norepinephrine, adenosine and prostaglandins) might hinder the activation of the sticky proteins by turning down the master switch.

To test that hypothesis, they studied cells from people infected with cytomegalovirus (CMV). T cells are supposed to seek out and destroy cells infected with CMV, but when patients' T cells were mixed with the suspect hormones in test tubes, the T cells' ability to activate the sticky proteins dropped.

Next, the researchers looked at what happened in people. Knowing that levels of these hormones naturally drop during sleep, they rounded up 10 healthy volunteers who were willing to spend one night snoozing in a sleep lab and another night, approximately two weeks later, awake in the same the same lab.

All of the volunteers had been infected with CMV, a mostly benign virus. "We recruited healthy humans seropositive for CMV because (they usually have) a high number of antigen-specific T cells," Dimitrov said in an email. That meant the researchers would have no trouble finding CMV-targeted T cells to study in the volunteers' blood, his team explained in the Journal of Experimental Medicine.

During the nights designated for sleeping, volunteers were hooked up to intravenous catheters, so researchers could draw blood samples without disturbing anyone's slumber.

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The researchers compared T cells collected on slumber-filled nights to T cells from waking nights and found, as expected, that when volunteers were sleeping, levels of stress hormones were lower than when volunteers stayed up all night. More important, T cells from sleeping nights had more infection-fighting sticky proteins activated than those from waking nights, meaning they were more potent.

Scientists have long known that lack of sleep can impact the immune system, said Dr. Louis DePalo, a professor of medicine, pulmonary, critical care and sleep medicine at the Icahn School of Medicine at Mount Sinai in New York City.

"Multiple clinical studies have demonstrated that people who do not get quality or sufficient sleep are more likely to get sick after being exposed to viruses," DePalo said in an email. "This (new) study demonstrates yet another molecular pathway where good quality and quantity sleep may lead to immune supportive effects via immune cells, called T cells."

DePalo, who was not involved with the new study, added that it "therefore presents another uniquely described mechanism underlying some of the immune supportive effects of sleep."