We may be in the most peaceful room with the most comfortable bedding, but for some reason when we sleep in an unfamiliar place, our sleep is often less than optimal. This phenomenon, known as the “first night effect," is very common. Researchers at the Department of Cognitive Science, Linguistics, and Psychology at Brown University appear to have solved the mystery. What causes us to have fitful sleep during the first night in an unfamiliar environment?

Trouble falling asleep and micro-awakenings, a decrease in REM sleep: we’ve all experienced these characteristics of fragmented sleep the first time we sleep in a new vacation rental or at a friend’s house. Until the publication of this study in Current Biology, no explanation had been put forth as to the origins of this "first night effect."

Through three experiments, Yuka Sasaki and colleagues observed the brain activity of 35 participants over the course of two nights using three techniques: MRI, EEG and MEG. The researchers focused most of their attention of slow wave activity, which enabled them to determine the depth of sleep. In the first experiment, 11 subjects (7 women, average age=23.6 years) spent two nights in a new place (the research center). In the second experiment, 13 subjects (7 women, average age=25.2 years) slept at the center, and during deep sleep, sounds were played at regular intervals, alternating between the left and right ear. The participants were made aware of the process beforehand and instructed to do nothing. In the third experiment, 11 subjects (9 women, average age=24 years), were subjected to the previous experiment, but were asked to react when they heard the sounds (by snapping their fingers). Each participant filled out a subjective questionnaire in order to assess their quality of sleep on a scale from 1 to 4 (1. Very good, 2. Good, 3. Not bad, 4. Very bad). What did we learn from these experiments?

During the first night, the researchers noticed particular activity in the left hemisphere, which disappeared during the second night. When sounds were played to the left ear (which is linked to the right hemisphere), the subjects generally remained asleep. But when sounds were played to the right ear (linked to the left hemisphere), the participants tended to wake up and react more quickly (experiment 3). These differences weren’t seen during the second night.

Thus, the “first night effect” can be explained by an interhemispheric asymmetry and the unique activity of the left hemisphere, which demonstrates a stronger connection to certain areas. This appears to act as a “night watch,” which makes us more vigilant and likely to react to risk factors when we spend a first night in an unknown (and potentially dangerous) environment. The study only analyzed the deep sleep phase. We don't know if this monitoring by the left hemisphere lasts all night, or if instead, the two hemispheres alternate. This hypothesis put forth by Y. Sasaki’s team draws on observations from marine mammals. Dolphins and whales sleep one hemisphere at a time, largely in order to return to the surface to breath.