Sleep loss weakens both arms of immune defense: it reduces the antibody response to vaccination and lowers natural killer cell activity, while raising inflammatory markers such as CRP and IL-6. The result is poorer protection against infection and a chronic low-grade inflammatory state.
The hepatitis A vaccine study
Healthy adults were vaccinated either after a normal night of sleep or after one night of sleep deprivation. The sleep-deprived group produced substantially fewer antibodies and had roughly half the proportion of participants reaching the protective threshold. The vaccine was the same; only the sleep before it differed.
Why the immune shift happens
Cortisol and catecholamines, both elevated under sleep restriction, suppress lymphocyte proliferation and shift the cytokine balance toward inflammation. At the same time, the normal nocturnal pulses of growth hormone and prolactin that support immune cell function are reduced. The net effect is weaker adaptive immunity and higher baseline inflammation.
The inflammatory elevation seen in sleep restriction is chronic and low-grade, not the acute response to an infection. It is associated with increased infection risk and poorer vaccine response, but it is not equivalent to being sick.