Individuals with Prader-Willi syndrome (PWS) are at risk for excessive daytime sleepiness (EDS) and disruptive behavior. This pilot study explores temporal characteristics of EDS and severe disruptive behavior across time of day and day of week in seven individuals with PWS (aged between 33 and 49 years) of whom five were matched to controls. Direct care staff and/or parents used a scatter plot (i.e., 2-h partial interval recording) to collect data during 28 successive days. Overall frequency of EDS and severe disruptive behavior was low in both groups. Individuals with PWS generally showed more EDS when there were no scheduled activities compared to when activities were scheduled, specifically in the afternoon and in the evening and during the weekend. Scatter plot methodology may be useful in identifying situations that evoke excessive sleepiness and severe disruptive behaviors in people with PWS. (Contains 4 figures.)