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Shortened sleep flattens mood and reduces reaction time, raising risk of accidents, study finds

Photo by Claudia Mañas on Unsplash
On the morning after their sleep was cut short, the study participants reported a decrease in their positive affect, which is the term psychologists use to describe positive emotions, such as cheerfulness, enthusiasm, joy and energy.

Sleeping just 90 minutes or so less than usual can have a significant impact on how quickly and accurately our brains function the next morning, raising our risk of making mistakes and having accidents, according to new research from Norway.

The study also found that cutting short our sleep by a relatively moderate amount of time has a very real effect on how we feel the next morning.

“Not in the sense that we have more negative feelings, like being down or depressed,” explains Ingvild Saksvik-Lehouillier, the study’s lead author and a psychologist at the Norwegian University of Science and Technology, in a released statement.

“Participants in our study experienced a flattening of emotions when they slept less than normal. They felt less joy, enthusiasm, attention and fulfillment,” she says.

This isn’t, of course, the first study to find that sleep deprivation negatively affects cognitive skills and emotions. Yet previous research on this topic has usually involved total sleep deprivation (staying up all night) when partial sleep deprivation (sleeping about an hour or so less than normal) is more common in everyday life.

Furthermore, studies that have looked at the effects of partial sleep deprivation have tended to be conducted in sleep laboratories, which may not accurately reflect how people sleep in real life. In addition, these studies usually restrict participants to the same shortened length of sleep — typically four to five hours. That approach doesn’t take into consideration people’s individual sleep needs.

To overcome those problems, Saksvik-Lehouillier and her colleagues designed a study that had participants shorten their sleep in a naturalistic environment (their homes), while at the same time controlling that experimental setting as much as possible by tracking the participants’ sleep with actigraphs (wrist-worn devices that keep track of when someone is awake and asleep, as well as when they are physically active).

How the study was done

For the study, the Norwegian researchers recruited 52 healthy young adults, aged 18 to 35. For the first seven nights of the 11-day study the participants slept for their usual length of time in their homes, although they wore actigraphs to measure their sleep patterns. They also kept sleep diaries.

For the next three nights, the participants were asked to sleep two hours less than normal. Again, their sleep patterns were tracked and measured. Those measurements showed that all the participants reduced their sleep by at least 90 minutes per night and, as a group, they averaged 124 minutes less per night.

On five of the mornings (three after the “normal” nights of sleep and two after the shortened ones), the participants took a 14-minute test that’s often used to measure people’s ability to react quickly and accurately to a visual task. The test, which is done on a computer, began about an hour-and-a-half after the participants awoke (and before they had any coffee).

The participants also filled out questionnaires designed to identify 20 positive and negative emotions.

What the study found

On each successive morning after sleeping normally, the participants improved their scores on the test. But after just one night of losing 90 to 120 minutes of sleep, their scores fell — and continued to fall after each of the final two nights.

“The reaction time went down after the participants had been sleep deprived, but the error rate went up,” says Saksvik-Lehouillier. “It seems that we react more quickly to compensate for lower concentration. Then there’ll be more mistakes.”

Interestingly, the participants were aware that they were doing more poorly on the test after not getting enough sleep and tried to compensate by putting more effort into getting the answers right. That extra exertion did not, however, bring their scores back up to “normal” levels.

On the morning after their sleep was cut short, the participants also reported a decrease in their positive affect, which is the term psychologists use to describe positive emotions, such as cheerfulness, enthusiasm, joy and energy.

“We didn’t find clear differences when it came to the negative emotions, but there were marked differences for the positive ones,” says Saksvik-Lehouillier. “Positive feelings scored worse after just one night of reduced sleep, and dropped even more after three nights.”

“I think this is a really interesting find,” she adds. “We already know that fewer positive emotions have a major impact on mental health. We also know that poor sleep is included in virtually all mental health diagnoses.”

Limitations and implications

A small number of people participated in the study, and they were all young, healthy Norwegian adults. The findings may not be generalizable. In addition, the study measured cognitive function and emotions only during the morning hours. It didn’t look to see if those effects lasted throughout the day.

Still, laboratory studies have shown partial sleep deprivation can have a negative impact on attention and other executive brain functions, as well as a worsening effect on mood.

All of this research underscores the need for good sleep practices. One in three American adults is sleep deprived — not regularly getting the recommended minimum of seven hours each night, according to the Centers for Disease Control and Prevention.

“It’s easy for us go to bed later than we should, especially when we think, ‘I just have to finish watching this series,’” says Saksvik-Lehouillier. “But we still have to get up to go to work, or study, or deliver our kids to kindergarten. This contributes to getting too little sleep.”

“How long we sleep is just part of the picture, but when we sleep is also important,” she stresses. “An irregular circadian rhythm can be worse than sleeping too little. Going to bed and getting up at the same time is recommended.”

FMI:  The study was published in the journal SLEEP, where it can be read in full. For more information about getting a good night’s sleep, go to the National Institute of Aging’s website. The tips are meant for older adults, but they apply to people of all ages.

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