"Springing forward" with daylight savings time may give us an hour more of sunlight in the summer, but it appears our internal body clocks never really adjust to the change, says a new study.

German researchers, reporting in the journal Current Biology, say the one-hour time adjustment of switching to daylight saving time can cause a significant disruption that has unintended effects on our bodies.

Just like other animals, the human internal clock -- called our circadian rhythm -- uses daylight to stay in synch with our environment as the seasons change, say the researchers. While previous research has generally suggested that sleeping patterns adjust within days of the time change, Till Roenneberg of Ludwig-Maximilian-University in Munich, says his research suggest that isn't so.

In a large survey he helped conduct, his team examined the sleep patterns of 55,000 people in Central Europe. His team concluded that when people are allowed to wake whenever they choose on their days off, their sleep timing follows the seasonal progression of dawn under standard time, but not under daylight saving time.

"While we generally think that the time changes enforced by the DST transitions are 'only an hour,' they have far more drastic effects if viewed in the context of the circadian clock's seasonal changes," Roenneberg said.

"When we implement small changes into a biological system which by themselves seem trivial, their effects, when viewed in a broader context, may have a much larger impact than we had thought."

In a second study, Roenneberg's team analyzed the timing of sleep and activity of 50 people for eight weeks around each of the two daylight saving time transition: the one in the spring and the one in the fall. The team took into account each individual's natural clock preferences, that is looking at whether they were "morning people" or "night owls."

They found that the timing of both sleep and peak activity levels easily adjusted to the end of daylight saving time in autumn, but that the timing of activity did not adjust easily to the start of daylight saving time in spring -- especially in "night owls."

"Essentially, their biological timing stays on standard, winter time, while they have to adjust their social schedules to the advanced clock time throughout the summer," says Roenneberg.

Roenneberg says this forced change on our circadian rhythms could have long-term effects on our health, but it is not yet clear what those effects might be. He suggests more research on the topic.