Knoxville (WVLT) -- Anyone who has ever worked the overnight shift knows it is hard to do.
Now, there is growing evidence it can be dangerous to your health.
Volunteer TV's Health Reporter Jessa Goddard has the details.
When the call comes in, at any hour, day or night, they've got to be ready.
Most of the time, firefighters don't get the luxury of 40 winks.
"I can work a full day no sleep at all, have gotten 2 hours of sleep pretty much wiped out after that," said firefighter Ryan Shelton.
The odd hours they keep may be another hazard of an already dangerous job.
Next month, the World Health Organization will identify working the graveyard shift as a probable cause of cancer.
The theory goes that overnight work disrupts the body's biological clock, interfering with the production of a hormone called melatonin.
"It could be disregulated or abnormally regulated in people who don't sleep during the night, so some people think that might be stimulating cancer growth," said Dr. Ronald Ennis, with St. Luke's-Roosevelt Hospital.
"i believe it, a lot of my night shift friends have high blood pressure cardiac and cancer," said Registered Nurse Sabrina Jackson.
In recent years, several studies focusing on nurses and airline crews have found women working at night were more prone to breast cancer, but larger studies are needed to confirm those findings.
There may be other factors contributing to higher cancer rates among night workers, including sleep deprivation.
Nurse Michotte Nabu worked the 10 p.m. to 10 a.m. shift for twelve years.
"You have rituals for going to sleep, but you are not actually sleeping 7 or 8 hours during the day, there are just so many disturbances," said Nabu.
Perhaps the best thing people working the night shift can do is sleep in a darkened room.
A dark "day's" sleep can trick the body clock and doctors believe it will keep night owls healthier.
If the graveyard shift theory eventually proves correct, millions of people worldwide could be affected.
Experts estimate nearly 20 percent of the working population in developed countries work night shifts.