Sleep that knits up the ravell'd sleeve of care, The death of each day's life.

-- from Macbeth

Shakespeare called sleep the "chief nourisher in life's feast," but scientists have yet to figure out why we need sleep every night.

Now an important step in unraveling the sleep mystery comes from the Chicago Medical School, where physiologist James M. Krueger has isolated a sleep-promoting substance from human urine.

Dubbed Factor S, it is part of a class of chemicals able to promote sleep, regulate temperature and boost the immune system. These chemicals -- called muramyl peptides -- can also lower blood pressure and are being pursued by drug companies as possible new blood pressure medication and powerful sleeping pills.

Factor S and related compounds seem to provide an important communication link between certain white blood cells, called macrophages, and the brain. His theory is that Factor S may be one way white blood cells tell the brain, "We've got a lot of bacteria here."

Evidence suggests that these chemicals are analagous to vitamins, since, as Krueger explains, "they're required by the body, but they can't be synthesized there."

The sleep connection may work like this: Bacteria seem to manufacture Factor S. When macrophages digest the bacteria, they release a chemical cousin of Factor S that travels through the blood to the brain. There it is absorbed and prompts certain brain cells to release interleukin-1, a substance that promotes sleep and stimulates the immune system.

Sleep may be an aid to the body, Krueger says, because it allows the immune system to recover from the "environmental challenges encountered during waking activity."