When we get sick it feels natural to try to hasten our recovery by getting some extra shuteye. Researchers from the Perelman School of Medicine at the University of Pennsylvania found that this response has a definite purpose, in fruitflies: enhancing immune system response and recovery to infection. Their findings appear online in two related papers in the journal SLEEP, in advance of print editions in May and June.

“It's an intuitive response to want to sleep when you get sick," notes Center for Sleep and Circadian Neurobiology research associate Julie A. Williams, PhD. “Many studies have used sleep deprivation as a means to understand how sleep contributes to recovery, if it does at all, but there is surprisingly little experimental evidence that supports the notion that more sleep helps us to recover. We used a fruitfly model to answer these questions.” Along with post-doctoral fellow, Tzu-Hsing Kuo, PhD, Williams conducted two related studies to directly examine the effects of sleep on recovery from and survival after an infection.

In the first paper, they took a conventional approach by subjecting fruit flies to sleep deprivation before infecting them with either Serratia marcescens or Pseudomonas aeruginosa bacteria. Both the sleep-deprived flies and a non-sleep-deprived control group displayed increased sleep after infection, what the experimenters call an "acute sleep response." 

Unexpectedly, the pre-infection, sleep-deprived flies had a better survival rate. "To our surprise they actually survived longer after the infection than the ones who were not sleep-deprived," notes Williams. The Penn team found that prior sleep deprivation made the flies sleep for a longer period after infection as compared to the undisturbed controls. They slept longer and they lived longer during the infection. Inducing sleep deprivation after infection rather than before made little difference, as long as the infected flies then got adequate recovery sleep. "We deprived flies of sleep after infection with the idea that if we blocked this sleep, things would get worse in terms of survival,” Williams explains. “Instead they got better, but not until after they had experienced more sleep."

Sleep deprivation increases activity of an NFkB transcription factor, Relish, which is also needed for fighting infection. Flies without the Relish gene do not experience an acute sleep response and very quickly succumb to infection. But, when these mutants are sleep-deprived before infection, they displayed increased sleep and survival rates after infection. The team then evaluated mutant flies that lacked two varieties of NFkB (Relish and Dif). When flies lacked both types of NFkB genes, sleep deprivation had no effect on the acute sleep response, and the effect on survival was abolished. Flies from both sleep-deprived and undisturbed groups succumbed to infection at equal rates within hours.

“Taken together, all of these data support the idea that post-infection sleep helps to improve survival," Williams says.

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