The group with delayed bedtimes did showed a link with negative moods. However, when compared with the delayed bedtime group, the forced-to-wake group participants had shorter periods of deep, slow-wave sleep. The lack of sufficient slow-wave sleep showed a statistically significant association with a reduction in positive mood—suggesting that sleep fragmentation is especially detrimental to a person’s positive mood. The interrupted sleep also reduced energy levels as well as feelings of sympathy and friendliness. Researchers also said the study suggests that the effects of interrupted sleep on positive mood can be cumulative, because differences between the two groups showed up after the second night and continued the day after the third night of the study. “You can imagine the hard time people with chronic sleep disorders have after repeatedly not reaching deep sleep,” commented one of the researchers.