We all know the feeling. It’s your first night at a hotel after a long day a travel. You slide under bleach-white sheets, collapsing on a cloud of pillows. Yet, despite near exhaustion, you toss and turn, unable to nod off.
This tendency to sleep poorly on the first night in a new setting, known as the “first night effect,” is well documented, but the causes have remained unclear.
This phenomenon, though, might be an evolutionary advantage in disguise, a new study in Current Biology suggests. The grogginess may happen because one side of the brain forgoes sleep to act as a “night watch” capable of alerting us to potential dangers, a team from Brown University shows.
“When a subject comes into a lab on the first night [for a sleep study], it takes them longer to fall asleep, they wake up many times in the middle of the sleep session, and the duration of deep sleep is shorter than usual,” says the study’s lead author, Masako Tamaki. “Usually researchers just throw away the data because the quality is so low, but we were curious what is going on in the sleeping brain on that first night.”
During sleep, a person’s brain journeys through a series of stages, each of which has a distinct electrical signature and is associated with a different depth of sleep. Tamaki and her team focused on the deepest form of sleep, called slow wave sleep, which is when we are most vulnerable. They started by inviting a group of subjects to sleep in the laboratory for two consecutive nights. Each participant was hooked up to several instruments that measured activity levels in four networks within each hemisphere of the brain.
On the first night, the amount of slow wave activity in the left hemisphere of the sleepers’ brains was significantly lower than in the right hemisphere. But the second night, the two hemispheres were similar, as has been seen in previous brain studies. These differences in deep sleep between the two hemispheres were most profound in the brain’s default mode network, several regions that are associated with daydreaming and other internal thoughts that occur while awake.
Based on these findings, Tamaki and her colleagues were curious whether that lighter sleep in the subject’s left brain would enable them to more closely monitor their environment for potential dangers, akin to what has been documented in animal studies. The researchers exposed a new batch of sleeping subjects to infrequent, high-pitched sounds mixed in with regular “beeps” presented every second during slow wave sleep. The sound patterns were played separately to both the right and left ear, each of which relays signals to the opposite hemisphere of the brain.
During the first night of sleep disturbance, the left hemisphere showed greater activity in response to the sounds than the right. These differences occurred only in response to the irregular sounds, which were designed to simulate something unusual and possibly dangerous. Once again, this hemispheric imbalance disappeared on the second night.
But did these neural differences actually cause people to wake up and react more rapidly? To test this, a third group was exposed to normal and abnormal tones while sleeping. The participants were asked to tap their finger when they heard a sound. On the first night, strange sounds presented to the right ear, which are processed in the left hemisphere of the brain, resulted in more awakenings and faster reaction times than those that were played to the left ear. A subsequent analysis showed that these reaction times were correlated with the amount of slow wave activity asymmetry in the brain. And as with each of the preceding experiments, the effects vanished the second evening.
“At some level, the brain is continuing to analyze things, even though you are not aware of the analysis,” says Jerome Siegel, director of the Center for Sleep Research at the University of California, Los Angeles. “If something unusual happens—if a door opens or you hear a key in a lock—you can alert to that, even thought the intensity of the stimulus is quite low.”
Researchers have documented such asymmetry in brain activity during sleep in birds, fur seals, dolphins and beluga whales, Siegel notes. In dolphins, for example, at least one brain hemisphere remains entirely awake and vigilant at all times, allowing the other half to safely descend into deep sleep. “The phenomenon is much more subtle in humans, but it is reasonable to expect that it would exist to some extent,” he says.
“Although our brain is very different from marine mammals and birds, we all need some technique to protect ourselves during deep sleep,” adds Tamaki. It could be that “our brains developed so that we only need a small part of the brain to work as a night watch.”
Tamaki and her colleagues suggest that the left hemisphere may be responsible for guard duty because the connections between the default mode network and other brain regions are relatively stronger on the left side. This might facilitate a quicker response to potential threats.
It’s also possible that the night watch responsibilities may shift throughout the night. “We only analyzed the first sleep cycle, but there are four or five sleep cycles in one night,” says Tamaki. “So the vigilant hemisphere may change over time.”
Tamaki and her team hope to investigate this possibility in future studies, as well as the influence of the first night effect on learning and memory. The findings may also provide a greater understanding of chronic sleep conditions such as insomnia. Insomniacs tend to sleep better in a new place, Tamaki notes.
There are ways we might be able to tone down the bark of our neural watchdog, such as carrying something that makes us feel comfortable and at home, but the best preventative strategy may simply be to plan ahead, Tamaki says. “If you have some important event, it’s better to not arrive the day before so you don’t have to suffer from the first night effect.”