When it comes to turning back the clocks on our devices, technology has us covered. Our smartphones automatically adjust.

But our internal clocks aren’t as easy to re-program. And this means that the time shift in the fall and again in the spring can influence our health in unexpected ways.

“You might not think that a one hour change is a lot,” says Fred Turek, who directs the Center for Sleep & Circadian Biology at Northwestern University. “But it turns out that the master clock in our brain is pretty hard-wired, ” Turek explains. It’s synchronized to the 24 hour light/dark cycle.D

Daylight is a primary cue to reset the body’s clock each day. So, if daylight comes an hour earlier — as it will for many of us this weekend — it throws us off.

“The internal clock has to catch up, and it takes a day or two to adjust to the new time,” Turek says.

Scientists have documented that the shift to daylight saving time in the spring, when we lose an hour of sleep, is linked to an increased risk of heart attacks and traffic accidents.

These studies are a reminder of just how sensitive we are to time and rhythm. Over the last 20 years, scientists have documented that, in addition to the master clock in our brains, every cell in our body has a time-keeping mechanism. These clocks help regulate important functions such as sleep and metabolism. And increasingly, there’s evidence that when our habits — such as when we eat and sleep — are out of sync with our internal clocks, it can harm us.