Turns out that one side of your brain stays ‘vigilant’ to protect you!
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Night Watch in One Brain Hemisphere during Sleep Associated with the First-Night Effect in Humans
d Interhemispheric asymmetry in sleep depth occurs for the first night in a new place
d This interhemispheric asymmetry occurs in the default-mode network
Masako Tamaki, Ji Won Bang, Takeo Watanabe, Yuka Sasaki
Tamaki et al. find that when humans sleep in a novel environment, the default-mode network in one hemisphere is kept more vigilant to wake the sleeper up as a night watch upon detection of deviant stimuli. The regional interhemispheric asymmetric sleep in a novel environment may play a similar protective role to that in marine mammals and birds.
d The less-asleep hemisphere shows increased vigilance in response to deviant stimuli
d One brain hemisphere may work as a night watch during sleep in a novel environment
Tamaki et al., 2016, Current Biology 26, 1190–1194 May 9, 2016 a 2016 Elsevier Ltd. http://dx.doi.org/10.1016/j.cub.2016.02.063
Night Watch in One Brain Hemisphere during Sleep Associated with the First-Night Effect in Humans
Masako Tamaki,1 Ji Won Bang,1,2 Takeo Watanabe,1 and Yuka Sasaki1,*
1Department of Cognitive, Linguistic, and Psychological Sciences, Brown University, 190 Thayer Street, Box 1821, Providence,
RI 02912, USA
2Present address: Cognition & Brain Science, School of Psychology, Georgia Institute of Technology, 831 Marietta Street NW, Atlanta, GA 30318, USA
We often experience troubled sleep in a novel environment . This is called the first-night effect (FNE) in human sleep research and has been re- garded as a typical sleep disturbance [2–4]. Here, we show that the FNE is a manifestation of one hemi- sphere being more vigilant than the other as a night watch to monitor unfamiliar surroundings during sleep [5, 6]. Using advanced neuroimaging tech- niques [7, 8] as well as polysomnography, we found that the temporary sleep disturbance in the first sleep experimental session involves regional interhemi- spheric asymmetry of sleep depth . The interhemi- spheric asymmetry of sleep depth associated with the FNE was found in the default-mode network (DMN) involved with spontaneous internal thoughts during wakeful rest [10, 11]. The degree of asymme- try was significantly correlated with the sleep-onset latency, which reflects the degree of difficulty of fall- ing asleep and is a critical measure for the FNE. Furthermore, the hemisphere with reduced sleep depth showed enhanced evoked brain response to deviant external stimuli. Deviant external stimuli detected by the less-sleeping hemisphere caused more arousals and faster behavioral responses than those detected by the other hemisphere. None of these asymmetries were evident during subsequent sleep sessions. These lines of evidence are in accord with the hypothesis that troubled sleep in an unfamil- iar environment is an act for survival over an unfamil- iar and potentially dangerous environment by keep- ing one hemisphere partially more vigilant than the other hemisphere as a night watch, which wakes the sleeper up when unfamiliar external signals are detected.
RESULTS AND DISCUSSION
Does sleep disturbance caused by an unfamiliar environment, that is, the first-night effect (FNE), have only negative effects?
It has been suggested that a function of partial sleep such as uni- lateral hemispheric sleep in some birds and marine mammals is a protective mechanism to compensate for risks during sleep [5, 6]. This led us to ask whether the FNE is involved in some type of interhemispheric sleep to be vigilant in one brain hemi- sphere in humans as a protective mechanism.
In experiment 1 (see Supplemental Experimental Proce- dures), we tested whether a regional interhemispheric asym- metry occurs with the FNE using an advanced neuroimag- ing technique that combines magnetoencephalography (MEG), structural MRI, and polysomnography (PSG) in the sleeping brain. We investigated slow-wave activity (SWA), which is a spontaneous brain oscillation (1–4 Hz) in non-rapid eye move- ment (NREM) sleep. The reason that we focused on SWA is that it is the only sleep characteristic that reflects the depth of sleep [9, 12] and is supported by cross-species studies of local sleep including those for mammals and birds. Since SWA in hu- mans originates in cortical regions including the brain network such as default-mode network (DMN) [10, 11, 13], we hypothe- sized that the regional interhemispheric SWA occurs in brain networks while the FNE occurs. To test this hypothesis, we measured SWA from four brain networks (including the DMN; Figure S1) during the first sleep session in which the FNE occurs and the second sleep session in which the FNE does not occur (Table S1). We conducted a four-way repeated-measures ANOVA on SWA with the factors being network, hemisphere (left versus right), sleep stage (slow-wave sleep versus stage 2 sleep), and sleep session (day 1 versus day 2). A factor of sleep stage was included because the strength of SWA, or sleep depth, should be different between sleep stages. If regional interhemispheric asymmetry of SWA occurs with the FNE in a certain network, this should manifest as an interaction among the factors. We indeed found the four-way interaction significant (F3,30 = 4.45, p = 0.011; Figure S1 and Table S2). The hemi- sphere 3 sleep session interaction was significant only in the DMN among the four networks during slow-wave sleep (F1,10 = 10.03, p = 0.010; Table S2). Further analyses (Table S2) indicated that SWA in the left DMN on day 1 was signifi- cantly smaller than SWA in the right DMN on day 1 (Figure 1A; t10 = 2.59, p = 0.027, d = 0.8) and was also significantly smaller than SWA in the left DMN on day 2 (t10 = 2.69, p = 0.023, d = 0.8). There was no significant difference between days in the right DMN (t10 = 0.97, p = 0.355, nonsignificant [n.s.]). SWA associated with K complexes did not show any hemi- spheric asymmetry (Figure S2).
1190 Current Biology 26, 1190–1194, May 9, 2016 a 2016 Elsevier Ltd.
Figure 1. SWA Asymmetry in the DMN dur- ing Slow-Wave Sleep in Association with the FNE
(A) SWA in the DMN during slow-wave sleep. The red bars show the left hemisphere, and the blue bars show the right hemisphere. The values are mean ± SEM. Asterisks indicate a significant dif- ference in the post hoc tests after the four-way repeated-measures ANOVA (*p < 0.05).
(B and C) Scatter plots for the asymmetry index of DMN SWA against the sleep-onset latency for day 1 (r = 0.68, p = 0.022) (B) and day 2 (r = 0.03,
p = 0.935, n.s.) (C). *p < 0.05. The correlation coefficient on day 1 was significantly different from day 2 (zpf10 = 1.99, p = 0.046).
See Figure S1 for SWA in other networks, details of ANOVA results, and details of the asymmetry index. See also Figure S2 for additional data on SWA.
We further examined the relationship between the degree of interhemispheric asymmetry of SWA in the DMN and the degree of the FNE. We obtained an asymmetry index for SWA strength ([left SWA right SWA]/[left SWA + right SWA]) for the DMN dur- ing slow-wave sleep (Figure S1). If SWA asymmetry for the DMN is associated with the reduced sleep quality in the first sleep ses- sion, the asymmetry index should be significantly correlated with the sleep-onset latency, which is a sensitive parameter for the presence of the FNE [3, 14]. A strong and significant negative correlation between these measures was found on day 1, but not on day 2 (Figures 1B and 1C). The correlations were signifi- cantly different between days.
To our best knowledge, regional asymmetric SWA associated with the FNE has never been reported in humans. Why was this not found in previous studies? First, visual inspection of PSG did not detect any hemispheric asymmetry in the apparent ampli- tude of SWA in the current data. Second, frequency analyses on sensor-space MEG failed to reveal regional hemispheric asymmetry of SWA (Figure S2). It may be difficult to detect the regional asymmetric SWA in a specific network such as the DMN in association with the FNE, unless sleeping brain activities are examined across different sleep sessions, hemispheres, and brain networks with high spatial resolution.
Is the regionally lighter sleep in the left hemisphere shown in the results of experiment 1 on day 1 related to higher vigi- lance to external signals? In experiment 2, we addressed this question using an oddball paradigm, where the amplitude of the brain response evoked by rare stimuli correlates with vigi- lance [15, 16] with a new group of subjects (see Supplemental Experimental Procedures). If one hemisphere is more vigilant than the other on day 1, the amplitude of the brain response should be larger in the hemisphere than in the other hemisphere on day 1. Both infrequent deviant and frequent standard beeps were presented every 1 s monaurally while subjects were asleep.
We conducted a three-way repeated-measures ANOVA on the mean amplitude of evoked brain responses with the factors of sound type, hemisphere, and sleep session during slow-wave sleep (Figure S3) in which regional asymmetric SWA was found in experiment 1. The results indicated that the mean amplitude of brain responses to deviant sounds was significantly augmented in the left hemisphere compared to the right on day 1 (Figure 2A; t12 =2.92,p=0.013,d=0.9),butnotonday2(t12 =1.37,p= 0.195, n.s.). The amplitude of the brain response to the deviant sounds in the left hemisphere was significantly reduced on day 2 (t12 = 3.18, p = 0.008, d = 1.0), while there was no significant
difference between days in the right hemisphere (t12 = 0.66, p = 0.523, n.s.). No significant difference was found in the brain responses to the standard sound between the hemispheres or be- tween sleep sessions (Figure 2B). Thus, hemispheric asymmetry in the brain responses was specific to the deviant sounds on day 1. These results indicate that the left hemisphere was more vigilant than the right when the FNE occurred. Hemispheric asym- metry in the brain responses to the deviant sounds was not found during wakefulness or stage 2 sleep (Figure S3).
Next, we found that enhanced vigilance in the left hemisphere resulted in more arousals. An arousal is defined as an abrupt and short shift of electroencephalogram (EEG) frequency . We counted how often arousals occurred per minute following a deviant sound during slow-wave sleep. Arousals occurred more frequently on day 1 than day 2 (Figure 3A). Given that an arousal was induced by a monaural deviant sound, we examined whether the arousal occurrence depended on the contralateral hemisphere to the ear to which the deviant sound was presented. Here, a trial in which a deviant sound was presented to the right (left) ear is called a left- (right-) hemisphere trial . The percentage of arousal occurrence following the left-hemisphere trials occupied more than 80% of the total arousals on day 1 and was significantly larger than chance (Figure 3B; Wilcoxon signed-rank test, z12 = 3.22, p = 0.001). The percentage of arousal occurrence following the left-hemisphere trials was significantly larger on day 1 than on day 2 (Figure 3B; z12 = 2.49, p = 0.013). However, this left-hemi- sphere dominance in the arousal occurrence vanished on day 2 (z12 = 0.32, p = 0.751, n.s.). These results indicate that the left hemisphere showed more arousals with deviant external stimuli than the right hemisphere during sleep on day 1 when the FNE occurred.
Does the vigilant hemisphere on day 1 produce faster behav- ioral responses to deviant external stimuli than on day 2? If the FNE plays a role as a protective mechanism such as a night watch rather than showing a merely disrupted sleep, a faster behavioral response should be generated from sleep upon the detection of deviant external stimuli in the first sleep session. In experiment 3, we asked a new group of subjects to lightly tap fingers when they heard sounds while they were sleeping, using an oddball paradigm similar to experiment 2. First, a larger number of subjects were woken by left-hemisphere trials than right-hemisphere trials on day 1 compared to day 2 (Figure 4A). Second, the reaction time from a deviant sound to tap was signif- icantly faster on day 1 than day 2 (Figure 4B). Third, this faster response on day 1 was mainly driven by the shorter time from
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