The quality of sleep you get may be more important than quantity, according to a new study led by researchers at Johns Hopkins Medicine.

The research suggests that awakening several times throughout the night is more detrimental to people's positive moods than getting the same shortened amount of sleep without interruption.

As reported in the November issue of the journal Sleep, 62 healthy men and women were randomly subjected to three sleep experimental conditions: three consecutive nights of either forced awakenings, delayed bedtimes, or uninterrupted sleep. Subjects were then administered a standard mood assessment questionnaire before bedtimes the following night to measure how strongly they felt a variety of positive and negative emotions, such as cheerfulness and anger.

Participants subjected to eight forced awakenings and those with delayed bedtimes showed similar low positive mood and high negative mood after the first night. Significant differences emerged after the second night: The forced awakening group had a reduction of 31 percent in positive mood, whereas the delayed bedtime group had a decline of 12 percent. The researchers said they did not find significant differences in negative mood between the two groups on any of the three days, which suggests that sleep fragmentation is especially detrimental to positive mood.

Polysomnography tests were administered while the subjects were sleeping to monitor certain brain and body functions and assess sleep stages. Compared with the delayed bedtime group, the forced awakening group had shorter periods of deep slow-wave sleep. The lack of sufficient slow-wave sleep had a statistically significant association with the subjects' reduction in positive mood, the researchers say. They also found that interrupted sleep affected different domains of positive mood; it reduced not only energy levels, but also feelings of sympathy and friendliness.

Patrick Finan, the study's lead author and an assistant professor of psychiatry and behavioral sciences at the School of Medicine, says the study also suggests that the effects of interrupted sleep on positive mood can be cumulative, since the group differences emerged after the second night and continued the day after the third night of the study.

"When your sleep is disrupted throughout the night, you don't have the opportunity to progress through the sleep stages to get the amount of slow-wave sleep that is key to the feeling of restoration," he says.

Frequent awakenings throughout the night are common among new parents and on-call health care workers, he says. It is also one of the most common symptoms among people with insomnia, who make up an estimated 10 percent of the U.S. adult population.