The first night spent away from home, for example in a hotel, is often of poor quality. Some people complain of having “slept with one eye open”. This is particularly problematic for business travelers or other people who have to make a lot of trips, such as athletes or artists. In a study, researchers from Brown University in the United States discovered the neurological reasons behind this particular phenomenon. The results showed that one of the 2 hemispheres of the brain remains more awake than the other during deep sleep, as if this hemisphere was ready to face problems, in other words in “alert mode” or on night watch.
The problem of the “first night” already known to researchers
The researchers already knew that the first night spent away from home was often problematic. In sleep tests carried out for research purposes in a hospital or laboratory, this first night is often not taken into account for studies and is considered an “adaptation night”. Participants therefore often have to sleep more than one night in the research setting for the tests performed to better reflect a night spent at home. For once, researchers at Brown University sought to better understand this first night of sleep, so problematic and particular, in order to better understand the causes.
The left hemisphere remains in a state of alertness
To arrive at their results, the researchers called on 35 volunteers. By measuring the cerebral activity of each participant during the first night of sleep away from home, they found that a particular network located in the left hemisphere remained more active than in the right hemisphere and in particular during deep sleep. When the researchers stimulated the left hemisphere with irregular noises broadcast in the right ear, it increased the participants’ likelihood of waking very significantly compared to noises broadcast in the left ear. In other sleep phases and in 3 other networks tested during the first night, they found no difference in alertness or activity in either hemisphere. In the 2nd night of sleep away from home, the researchers observed no significant difference between the two hemispheres during deep sleep. The proof that it is truly the first night of sleep spent away from home that poses a problem. The researchers recognize that some questions remain unanswered, particularly on the effect of the right hemisphere, the rest of the night, that is to say in other phases of sleep, because the researchers observed only the first deep sleep phase. Because of this, they don’t know if the left hemisphere maintains its alert level all night, or might work alternately with the right hemisphere later in the night. Moreover, scientists do not yet understand why only one hemisphere remains on alert and not the other.
Animals that only sleep with one eye open
This research work has shown that when we find ourselves in a new environment, an asymmetry between the two hemispheres manifests itself at the level of deep sleep, vigilance and reactivity, like a night watch to protect us. This is common in some animals. Some sea animals and birds have been known to exhibit an asymmetry during sleep, to enable them, in particular, to remain alert throughout the night. This hemisphere-to-hemisphere asymmetry in deep sleep seen on the 1st night of sleep has never been reported in humans.
Masako Tamaki: Night Watch in One Brain Hemisphere during Sleep Associated with the First-Night Effect in Humans. Current Biology. DOI: https://doi.org/10.1016/j.cub.2016.02.063