题目选项分析

People who consume caffeine within six hours of going to sleep are shown to have less restful sleep than those who abstain from caffeine. Short daytime naps usually improve performance and alertness in adults. However, study subjects who consume caffeine immediately before a short afternoon nap performed better on tests and felt more alert after the nap than others who simply took naps.

Which of the following, if true, best reconciles the apparent discrepancy described above?

【选项】The naps were not long enough to have beneficial effects for either group.