Recently, the United States undertook the annual ritual of setting the clocks forward for Daylight Saving time, a day many people dread. Not only is an hour lost from a precious weekend day, but many people report feeling tired, stressed, or irritable as well. In other words, those affected are experiencing minor jet lag. Oddly, setting the clock backwards is not usually so painful. So what is jet lag, and why does it affect us more when we set the clock forward in the spring?

One of the best ways to examine the impacts of time shifts is, to eliminate time as a variable. Writing in The British Medical Journal, Josephine Arendt and Vincent Marks describe how volunteers were placed in darkened bunkers without clocks, deprived of light and social cues that tell us when to eat, sleep, or be merry.

Through constant measurements both before and after the deprivation, the researchers determined that under typical circumstances most people’s bodies function on a roughly 23 hour clock for hunger, energy level, sleeping, and other bodily functions. The cycle is called a circadian rhythm. Deprived of context, the volunteers’ body cycle stretched out into 25 hours. This may explain why losing time by traveling east or setting the clock ahead is so painful; we naturally slip into a longer cycle when our cues are mixed up, allowing us to cope with extra time far more easily than with losing it.

In practical terms, what does a 23 hour cycle mean? All our feelings—waking up, hunger, etc. are determined by hormones, so when our cycle is disrupted those hormones are secreted at all the wrong times. A classic example is cortisol, an adrenal hormone that is secreted in response to stress and low blood sugar. Most importantly for jet lag, it plays a role in our sleep cycle, helping us to wake up and be ready for the day, peaking in the body around 8am. Let’s say you travel from New York to Paris, which have a five hour time difference. The adrenal gland thinks it is 8am, and therefore time to ramp up the cortisol! Alas, it’s actually 3am. Now you’re awake and hungry at a completely inappropriate time, and there is nothing you can do about it. Voila! You are now jet lagged.

Since our body’s functioning is basically controlled by hormones, severe disruption in this system can have profound consequences beyond just feeling groggy. Medications taken while traveling, for example, might not have their typical impact. Long-term disruptions, like those experienced by shift workers or flight attendants, can go from annoying to deadly—long-duration shifts to circadian rhythms were officially classified as carcinogenic in 2007. At the moment there is not much to be done, but the takeaway is that jet lag has real physiological consequences.

Editor’s note: This article was updated to correct “Daylight Savings Time” to “Daylight Saving Time.”