This past weekend, I attended a scientific meeting in New York. As often seems to happen to me in a hotel, I tossed and turned and woke up feeling not very rested. The second night I did a bit better. Why is this? Using advanced neuroimaging techniques to study volunteers in a sleep lab, NIH-funded researchers have come up with a biological explanation for this phenomenon, known as “the first-night effect.”
As it turns out, the first night when a person goes to sleep in a new place, a portion of the left hemisphere of his or her brain remains unusually active, apparently to stay alert for any signs of danger. The new findings not only provide important insights into the function of the human brain, they also suggest methods to prevent the first-night effect and thereby help travelers like me in our ongoing quest to get a good night’s sleep.
The study, presented in a recent issue of the journal Current Biology, was led by Yuka Sasaki at Brown University, Providence, RI . In the first experiment, Sasaki and colleagues recruited 11 young and healthy people with normal sleep habits to monitor slow-wave brain activity, a characteristic that reflects the depth of sleep. For three days before the start of the study, participants stuck to their usual sleep schedules, avoiding alcohol and any unusual activity.
The researchers monitored each participant’s brain activity for up to three hours on the first and second nights in the sleep lab on two separate occasions. That allowed them to record brain waves and eye movements, and they could track each person’s stage of sleep. They also used an advanced neuroimaging method called magnetoencephalography (MEG) to measure slow-wave brain activity. Because MEG isn’t well suited to identify detailed brain structures, the researchers also used functional magnetic resonance imaging (fMRI) to examine the brains of study participants while they were awake.
Sasaki and colleagues focused their attention on four brain regions and found something intriguing. During the first night of sleep, a portion of the brain that the researchers call the default-mode network (DMN) showed a higher than normal level of activity on the left side, suggesting it was sleeping less deeply. The DMN has been described as an interconnected neural network that spans several regions of the brain. It is said to be associated with wakeful rest, such as daydreaming and just letting our minds wander.
That a portion of the brain might serve as a “night watch” isn’t without precedent in nature. In some animals, including marine mammals and some birds, half of the brain can be asleep, while the other half remains alert. Sasaki says the new findings suggest that the DMN might have a similar ability, although to a lesser extent than in animals, to disengage from the primary task of sleeping and react quickly if needed.
Interestingly, this increased activity in the DMN occurred only in the brain’s left hemisphere. In two follow-up experiments, the researchers asked whether this lighter sleep in the left side of the DMN allowed people to be more vigilant and responsive to high-pitched beeping alarms played against a background of faint beeps that they’d been instructed to ignore. And, indeed, it did. A new group of 13 study participants showed bigger jumps in brain activity in response to those alarms played to their right ears (corresponding to activity in the brain’s left hemisphere) on the first night of sleep compared to the second. In a third experiment, another new group of 11 people were instructed to tap their fingers when a sound woke them. Those individuals woke up enough to tap more often and more quickly on the first night compared to the second.
All three experiments demonstrated a similar pattern of disturbed sleep on the first night in the lab compared to the second. The asymmetry in activity between the left and right brain hemispheres might explain in part why researchers had never noticed this role for the DMN. This phenomenon has likely also been missed because researchers often deal with the first-night effect by simply throwing out that early data.
Sasaki says she is now curious to know whether this disruptive DMN activity can be knocked out to improve the first night of sleep using transcranial magnetic stimulation (TMS), a non-invasive method of brain stimulation sometimes used to improve mood and treat pain, or some other means. She’d also like to look for similar asymmetries in the brain activity of people who suffer from sleep disorders, including insomnia.
 Night watch in one brain hemisphere during sleep associated with the first-night effect in humans. Tamaki M, Bang JW, Watanabe T, Sasaki Y. Current Biology. 2016 May 9;26:1-5.
Your Guide to Healthy Sleep (National Heart, Lung, and Blood Institute/NIH)
Yuka Sasaki (Brown University, Providence, RI)
NIH Support: National Institute of Mental Health; National Eye Institute