Feeling sleepy? Shhh! Bacteria at work

UPDATE FEELING SLEEPY? SHHH! BACTERIA AT WORK

It's common knowledge that the sick get drowsy, but the reason isunexpected: bacteria may trigger deep sleep. In 1913 a group of French researchers took the cerebrospinal flu- id of sleep-deprived dogs and injected it into the brains of well rested dogs. The result: the unsleepy dogs slept. Some 50 years later, when Harvard re- searchers repeated the experiment with goats and rabbits, the same thing happened. Subsequently, they isolated a sleep-promoting substance, which they named factor (for sleep), and re- vealed its identity. It was a muramyl peptide, one of the building blocks of bacterial cell walls.

How exactly do bacteria induce sleep? Harvard biochemist ManfredKarnovsky believes that when the bac- teria that normally (and harmlessly) live in the gut are broken down by enzymes, the muramyl peptides from their walls are released. The free muramyl pep- tides may head for the brain and turn on the sleep switch themselves, but, more likely, they trigger sleep indirectly. In the latter case, they would bind to im- mune cells in the blood stream or to as- trocytes in the brain, causing these cells to pump out interleukin-1, a substance that's known to stimulate the immune system and to trigger slow-wave (non- dreaming) sleep.

The idea that bacteria induce sleep is borne out by the sleepingpatterns of newborn babies. For the first few weeks, infants have no slow-wave sleep, only dreaming sleep. Some scientists believe this is because dreaming sleep is essential to learning, and thus that new- borns, who have a lot to learn, have a lot to dream about. But Karnovsky thinks it may be because newborns have rela- tively sterile guts: the bacterial flora that live in the adult gut don't take hold until a few weeks after birth.

Although sleep may be induced nor- mally by the bacteria that inhabit the gut, the same effect may be precipitat- ed by bacteria that invade the body during an infection, says James Krue- ger, a physiologist at the University of Tennessee in Memphis. That may ex- plain why when you get sick you get extra sleepy.

It may also explain why it's good to give in to the sleepiness. Since sleeping and the immune response are intimate- ly connected -- both are stimulated by muramyl peptides and interleukin-1 -- one may goad the other. ''When your grandmother says you won't get sick if you sleep, she suspects that sleep is ben- eficial for your body's defenses,'' says Krueger. ''But no one has ever really tested her suspicion until now.''

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