David Spiegel of Stanford University has suggested that an out of line circadian rhythm can be linked to a risk of cancer:

Spiegel suggested two possible ways in which the circadian rhythm may influence cancer progression. The first involves a hormone called melatonin, which the brain churns out during sleep. Melatonin belongs to a class of compounds called anti-oxidants that mop up damaging free-radical compounds. With a disrupted circadian rhythm, the body produces less melatonin and the cell's DNA may be more prone to cancer-causing mutations.

Melatonin also slows the ovaries' production of estrogen. For many ovarian and breast tumors, estrogen spurs the cancerous cells to continue dividing. Shift workers who work through the night and produce less melatonin may therefore produce more cancer-activating estrogen, the researchers said.

The second link lies with a hormone called cortisol, which normally reaches peak levels at dawn then declines throughout the day. Cortisol is one of many hormones that help regulate immune system activity, including the activity of a group of immune cells called natural-killer cells that help the body battle cancer.


So flying internationally knocks out your circadian rhythm significantly too. Sleep better and your immune system and endocrine systems are healthier. As they say - tiredness can kill. This tallies with the study from a while back about cancer risks for nurses working night shifts (ie messed up circadian rhythms):
British Medical Journal
Science Daily
allnurses.com