Why You Get Poor Sleep in Unfamiliar Places and Wake Up Groggy on the First Day of Vacation

The accommodations may have been impeccable at your hotel: a Posturepedic bed with 1,200-thread-count Egyptian linen cotton sheets, blackout curtains and temperature control that kept your room consistently at an ideal 67 degrees. You snoozed a solid eight or nine hours, but judging from the way you feel the next morning, it's as if you barely slept a wink.

Blame evolution: A study published April 21 in Current Biology finds that when people sleep in an unfamiliar place, one hemisphere of the brain stays more awake as a way to keep watch for potential danger possibly a holdover from the days when Homo sapiens had to guard their territory every night.

This phenomenon is known as unihemispheric slow-wave sleep, and it's seen in marine animals and some birds. This is the first study to suggest that the human brain may also be hard-wired to function in a similar way, albeit on a smaller scale. Humans, unlike sparrows, don't usually sleep with one eye open. However, when in new surroundings, half the brain may remain at least a little bit awake—great for waking quickly if an intruder shows up, but with a resulting groggy feeling the next morning.

Sleep doctors often toss out the data from a patient's initial night in a sleep lab, what's known in the business as the "first-night effect." This group of researchers wanted to know what was actually causing this effect.

The group of researchers from Brown University's School of Cognition and Brain Science and School of Psychology recruited sleep study participants, and conducted neuroimaging along with polysomnography, a standard test used in sleep labs to monitor brain waves, oxygen level in blood, heart rate, breathing, and eye and leg movements.

They discovered that only the brain's right hemisphere was consistently engaged in slow-wave, or deep, sleep. The left hemisphere—the side responsible for logical thinking and reasoning—had what the researchers called "enhanced vigilance," which also made the entire brain more responsive to sound.

The researchers tried a test where they targeted sounds to the left and right ear. They found that on the first night, 80 percent of the arousals from deep sleep occurred when sound was made to target the right ear (the brain's left hemisphere). On day two, that number dropped to about 50 percent.

Next, the researchers asked a new group of participants to tap their fingers when they heard sounds through the night. A larger number of participants were woken up by sound in the right ear the first night compared to the second. The researchers also found the reaction time was faster on the first day.

The researchers say they're now working on using transcranial magnetic stimulation (TMS)—a type of therapy in which a small electric coil is applied to a certain area of the head to target a specific region of the brain depending on the therapeutic need. It's currently used sometimes to treat neuropathic pain, migraines and depression. If it works for sleep, there could be some hope for weary travelers. Instead of on-demand movies, the future's hotel guests could request TMS.