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 promoting sleep.

BBC: News | Health | Immune system blamed for Chronic Fatigue Syndrome

"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.

CNN: How to fix health problems with exercise

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.

FORBES: When Doctors Go Off-Label

While the science is in the early stages, one hypothesis suggests that by promoting feelings of safety and security, shared sleep in healthy relationships may lower levels of cortisol, a stress hormone.

WSJ: Who Sleeps Better at Night?