Immunologists and sleep researchers have tried to put this one to the test. Anecdotal evidence from people's personal experiences suggests a link between sleep and susceptibility to illness. Studies do suggest that the quality of your sleep prior to infection factors into whether you catch a cold and, if so, how severe it is. Some studies even suggest that those who get six hours or less of sleep have 50 percent less immunity protection than those who get eight hours per night. Certain cells that modulate the immune system increase during sleep, so missing out on sleep means your resistance to viral infection drops. And, in fact, researchers have found that when recipients are sleep deprived, the flu shot is less effective. If sleep weren't so key to immune health, then we'd probably not experience that compelling need to sleep when we're coming down with an illness.