Quantification of Sleep Fragmentation Through the Analysis of Sleep-Stage Transitions

Study Objectives: We introduce new quantitative approaches to study sleep-stage transitions with the goal of addressing the two following questions: (i) Can the new approaches provide more information on the structure of sleep-stage transitions? (ii) How does sleep fragmentation in patients with sleep apnea affect the structure of sleepstage transitions? Design: We analyze hypnograms and compare normal subjects and sleep apnea patients using numerous measures, including the percentage of sleep time for each stage, probability distributions of the duration of each stage, the sleep-stage transition matrix, and a measure of the asymmetry of this matrix. Setting: N/A Subjects: 197 normal subjects and 50 obstructive sleep apnea patients recruited in the SIESTA project. Results: We find that the time percentage for wake stage is identical for sleep apnea subjects and for normal subjects, but that the sleep apnea group have a faster decaying distribution of wake duration. Both normal subjects and sleep apnea patients have exponential distributions of duration for all sleep stages and a power law for the wake stage. We also find that there is a loss of preference of transition paths of sleep stages in sleep apnea. Conclusions: The new approaches proposed here enable us to show that the distribution of sleep and wake duration have different functional forms, indicating fundamental differences in the dynamics between sleep and wake control. The difference remains even in the fragmented sleep of sleep apnea. The fragmentation of sleep in sleep apnea results in a shorter wake duration and interrupts the structure of sleep-stage transitions of sleep apnea subjects, causing the loss of certain particular transition paths.