Changing the clocks twice a year for Daylight Saving Time throws off the body's sleep rhythms for 20 percent of the year, with potentially serious immune consequences, according to a study conducted by researchers from the Ludwig-Maximilians University in Munich, Germany.

Researchers studied the sleep patterns of 65,000 people living in Europe, noting at what time the body entered the "mid-point" of sleep, defined as halfway between falling asleep and waking up on a day off.

Normally, the body adjusts to changing daylight during different seasons by shifting the mid-point. The researchers found that when the clocks were set forward by an hour in the spring or back in the autumn, this natural progression was disrupted.

"If we are getting up in winter, the sun rises after we have gotten up," said lead researcher Tiss Roenneberg. "Then we are progressing into spring and for some days, we are getting up with the sun. Then in the following weeks after that, the sun rises before we get up."

This process "abruptly stops" with the Daylight Saving time change, the researchers found.

"With the DST change in spring, we suddenly have to get up again before the sun rises. This little one-hour time change throws our annual trajectory back by four weeks," Roenneberg said. "In autumn, it is even worse: It is thrown back by six weeks, so on the seasonal scale the one little hour perturbs the system by 10 weeks, a significant 20 percent of the entire year."

This disruption of sleep patterns can interfere with the body's immune system, which normally adjusts itself to the different pathogens that are prevalent at different seasons.

"Biological internal timing systems have evolved to accommodate and anticipate environmental changes, so they can react appropriately and proactively," Roenneberg said. "If we are perturbing these systems, we could potentially prevent our seasonal adaptation to be really effective. In general, the consequences may be minor, but if you add them up they could lead to enormous health costs."