Hypnograms before and after rapid palatal expansion. This figure presents the sleep stage architecture (sleep stage versus time: a hypnogram) of our patient’s sleep before and after rapid palatal expansion. Sleep stages increase in depth along the ordinate from Wake to non-rapid eye movement stages 1, 2 and 3 (N1, N2 and N3) to rapid eye movement. The time of night is represented on the abscissa. During the baseline polysomnogram, our patient fell asleep quickly and slept relatively continuously throughout the night with frequent shifts between deeper and lighter sleep/wake (sleep stage shifts; a total of 70 during the night). Frequent sleep stage shifts are thought to be an adaptation to sleeping under stress (such as the presence of danger or unfamiliar surroundings; see ). After rapid palatal expansion, our patient had difficulty falling asleep in the unfamiliar surroundings and slept fitfully for half the night with frequent sleep stage shifts (32 during the first half of the polysomnogram). This occurrence may reflect decreased sleepiness together with the stress of his unfamiliar surroundings (at baseline, he may have been too sleepy and too stressed for the unfamiliar surroundings to matter). Beginning at approximately 3:00 a.m., our patient became acclimated to his surroundings and went into deep sleep with few sleep stage shifts compared to his baseline study at the same time of night (38 during the baseline study compared to 19 after rapid palatal expansion). The finding of decreased sleep stage shifts during the second half of the second polysomnogram may be one marker for decreased vigilance during sleep reflecting decreased stress, after rapid palatal expansion.