Waking up tired is a common complaint for those who sleep away from home, and the effect is stronger on the first night. Now we know why: under these conditions, one hemisphere of the brain remains alert instead of relaxing, according to a study from Brown University in the United States (Current Biology, April 21, 2016). To understand the so-called first-night effect, the researchers used neuroimaging techniques and polysomnography to examine cerebral activity in 35 people who slept in the laboratory for a few nights. The images revealed that on the first night, the two hemispheres of the brain showed different activity patterns. The stages of sleep in one hemisphere were not as deep as in the other. The more awake hemisphere reacted sooner to external sounds. The degree of asymmetry was related to difficulty in achieving deep sleep on the first night. There was no asymmetry on the following nights. The initial pattern, which enables a person to remain alert in a strange environment, represents a moderated version of what is observed in marine mammals and some birds: they disconnect one hemisphere while sleeping. To reduce the first-night effect, the researchers suggest that people try to create an environment similar to the one in which they sleep at home.
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