Researchers found that people with CFS produced unusually high levels of the hormone melatonin, which stimulates the immune system and plays an important role in promotingsleep.
"We aren't sure why activity primes your body for sleep so well, but it's likely a combination of factors, including lowering your core body temperature, increasing the sleep-promoting hormone melatonin and supporting a biological need to restore energy levels and repair cells and tissues when you sleep, " Cardinal says.
For instance, AstraZeneca's (nyse: AZN - news - people ) Seroquel is often used for its sleep-promoting or calming effect. (see: " Antipsychotic Prescribed as Sleeping Pill") Eli Lilly 's (nyse: LLY - news - people ) Zyprexa has even been used in rare cases in patients with anorexia--one of the drug's potential side effects is weight gain.