An Excerpt from "Why We Sleep"

"If you compare the electrical depth of the deep NREM slow brainwaves on one half of someone's head relative to the other when they are sleeping at home, they are about the same. But if you bring that person into a sleep laboratory, or take them to a hotel -- both of which are unfamiliar sleep environments -- one half of the brain sleeps a little lighter than the other, as if its [sic] standing guard with just a tad more vigi­lance due to the potentially less safe context that the conscious brain has registered while awake. The more nights an individual sleeps in the new location, the more similar the sleep is in each half of the brain. It is perhaps the reason why so many of us sleep so poorly the first night in a hotel room." -- https://delanceyplace.com/view-archives.php?p=3549