Even if you’re not visiting a new time zone, sleep doctor Daniel I. Rifkin, M.D., explains that traveling can affect your sleep due to what’s known as the first-night effect, or FNE.
Physicians first came up with the concept when studying clinical data from polysomnography (sleep studies). They noted that the first night of sleep wasn’t necessarily indicative of a person’s true sleep habits. It took patients at least one night to ease into their new sleep setup.
Rifkin says that this idea can be applied outside the sleep lab, too. Any time you’re sleeping in a new or unfamiliar environment, it’s safe to say you’ll have a longer sleep onset time—aka it will take you longer to fall asleep. “You’ll also forgo your first rapid eye movement (REM) period in many cases,” he adds.
This disruption occurs because our brains are busy taking stock of any potential threats in our new environment—even if we’re not consciously aware of it. “Our subconscious brain knows that’s not our normal place, so there’s probably a little bit of heightened awareness,” says Rifkin.
In one fascinating 2016 study out of Brown University, researchers found that the left brain hemisphere tends to be more vigilant than the right hemisphere whenever we sleep in new surroundings. (This brain splitting is something a lot of animals do, but this was the first time it was demonstrated in humans!) The greater the difference between these two hemispheres, the longer it will take for you to fall asleep.
While the Brown study found that the brain typically adjusts after the first night in a new environment, you may find it takes you some more time to get back on track—particularly if you’re traveling somewhere that has an uncomfortable bed, a noisy room, or any of the many other sleep disrupters.
Since the first-night effect can keep you awake for longer and eat into a cycle of your REM sleep—which is essential for memory consolidation—you may find yourself feeling a little foggy-headed on your first few mornings at your destination.