You are right and your explanation is more precise. The fact there was no cognitive or other change due to the 27 extra minutes implies my interpretation. It's

You are right and your explanation is more precise. The fact there was no cognitive or other change due to the 27 extra minutes implies my interpretation. It's possible that the baseline using 5.5 hours of sleep is not the right one to use and 8 hours of sleep might show a positive impact, which the extra 27 minutes is not showing.