Most people will have experienced the so-called first-night effect at some point in their life. When sleeping in an unfamiliar setting for the first time, it is likely to take longer to drop off, and the sleep we finally do get will be broken and unsatisfying. For the first time, scientists have discovered why this might be. Share on Pinterest What drives the so-called first-night effect? Although the first-night effect (FNE) has been part of the human experience since humans first slept in a new cave, the science behind it has remained a mystery. The FNE is so familiar and consistent that sleep researchers routinely discard the first night of data, as they know it will be unusual and, therefore, unusable. Researchers from Brown University in Rhode Island, led by Yuka Sasaki, set out to investigate the odd phenomenon in more depth. The team wanted to know why the FNE occurs, and whether there is any benefit to it. Using advanced neuroimaging techniques to take snapshots of the sleeping brain, the team built up a detailed picture of the sleep activity during the first night in a new location. Measurements included magnetoencephalography, structural MRI (magnetic resonance imaging), and polysomnography (measuring blood oxygen levels, breathing and heart rate, eye and leg movements). Slow wave sleep, rather than REM (rapid eye movement) sleep, was the main parameter that the team focused on because it acts as a direct measurement of the depth of an individual’s sleep.

The left-right split Sasaki and her team were surprised by the results. They found that during the first night of sleep, the left side of the brain was significantly less asleep than the right; the two hemispheres were not asleep in equal amounts, they displayed observably different patterns. One of the primary measures of the FNE is the length of time an individual takes to get to sleep; this was shown to be dependent on the degree of asymmetry between the hemispheres. In other words, the more different the two sides of the brain behaved, the longer it took for an individual to nod off. A second leg of the experiment showed that the left hemisphere was more sensitive to external sound stimuli during sleep; not only would the brain produce a larger response in reaction to a random noise, but the participant was also more likely to be aroused. When the team measured the same individuals on the second night, those sensitivities to sound in the left brain vanished.

What are the benefits of the FNE? Having a disturbed sleep pattern can make the next day challenging, fighting through the morning meeting carrying heavy eyelids and a vat of coffee. What benefits could this have? It turns out, human brains are not the first to have developed such a habit. Other animals are known to sleep with half of their brain alert, marine mammals and some birds, for instance. This kind of hemispherical asymmetry, referred to as unihemispheric slow-wave sleep, allows part of the brain to remain vigilant; if there is a strange sound, we are more likely to be aroused and ready for danger. Birds are able to sleep, one hemisphere at a time, literally keeping one eye open for predators. Some scientists believe that certain bird species can snooze while on the wing, during long migratory flights. As evidence for the increased vigilance hypothesis, in a third experiment, the team asked participants to tap their fingers lightly if they heard a sound while sleeping. Sasaki found that on the first night of sleep, compared with the second, the participants were more likely to respond, and, when they did respond, it was significantly faster. Although our brains do not show the same degree of hemispheric variation as dolphins, for instance, Sasaki says: “We know that marine animals and some birds show unihemispheric sleep, one awake and the other asleep. […] Our brains may have a miniature system of what whales and dolphins have.”