Fig. 1.

Illustration of survival curve analysis of NREM sleep. Step 1 depicts an example of sleep over time. For reasons of clarity, only one sleep cycle is depicted. During step 2, all fragments of NREM sleep are listed and sorted based on length. Fragments of NREM sleep are assumed to be terminated if they are followed by either wake or REM sleep. In step 3, NREM fragments shorter than 1 min are excluded from the analysis. On the right, the hazard rate resulting from these sleep fragments is plotted. The survival rate represents the percentage of NREM fragments longer than a certain length, e.g., after 1 min, three (60%) out of five of the sleep fragments are still left (i.e., “has survived”), and after 5 min, only one (20%) sleep fragment is left. A Weibull curve was fit to the hazard rate (not indicated in the figure). Survival curves of REM sleep and wake sleep were calculated using a similar approach. This illustration is a simplification. In reality, we have four sleep cycles and thus more sleep fragments