We’ve known for years that poor sleep and depression are inextricably linked. Just observe your mood after getting a bad night’s sleep: You can feel cranky, anxious, and fatigued — all symptoms that are also associated with depression.
But exactly which aspect of an unsatisfying snooze causes a plummeting mood the next day has been harder to parse.
What makes you crankier: getting fewer hours of sleep or getting a full eight hours with periodic interruptions?
A new study published in the journal Sleep has taken a crack at answering this question, and the findings suggest that the latter condition may actually be worse. Repeatedly waking up throughout the night may be harder on your mood than getting less sleep.
These results were observed in a small group, so more research is still needed. An earlier study, also small and with slightly different conditions, found that people who were awakened four times in eight hours experienced sleep problems that were comparable to — not worse than — people who slept for just four hours. (Those subjects were awakened only half as many times as in the new study.)
The preliminary findings of the 2015 study in Sleep, which allowed a more exact head-to-head comparison of restricted versus interrupted sleep, offers more evidence demonstrating that a night of tossing and turning can cause real harm, even if you’re in bed early.
In fact, if you’re concerned about your mood, staying up late may be a better option than waking up many times throughout the night. But nothing beats sleeping soundly for a full night.
Three different kinds of sleep
To test how sleep quality affects a positive mood, the researchers split a cohort of 62 healthy men and women into three groups of different sleep situations — uninterrupted sleep, forced awakening, or delayed bedtime.
The lucky control group of 24 subjects slept normally: They had the opportunity to sleep for a full eight hours for three consecutive nights.
The second group of 21 subjects had the same amount of time to sleep as the group above, but would periodically be awakened by a nurse eight times over the course of the night. For each awakening — one lasted an hour, and the rest lasted 20 minutes — participants were forced to stay awake while sitting up with the lights on. The longest each of these participants were able to sleep in total was 4.7 hours. This is called the interrupted sleep group.
The 17 people in the third group were allowed exactly the same amount of sleep time that their assigned partner in the interrupted group got. For example, if someone in the interrupted group only slept for a total of 3 hours, their assigned partner would only be allowed 3 hours of total, uninterrupted sleep. This is called the delayed sleep group.
Participants in the interrupted and delayed sleep groups were continuously monitored via polysomnography, which records brain waves, blood oxygen levels, heart rate, breathing, and eye and leg movements during the full eight-hour “sleeping” period.
To test how interrupted and delayed sleep times affected the subject’s positive mood, they had the participants fill out a mood assessment questionnaire every night before going to bed. It asked how strongly subjects felt a range of positive or negative emotions, such as friendly and sympathetic or annoyed and angry.
After analysing the results, the team found that while the negative moods of both the interrupted and delayed sleep groups became worse at about the same rate over the three days, the positive moods of those in the interrupted sleep group plummeted more significantly than those of the group with delayed sleep.
In other words, getting interrupted throughout the night is a bigger hit to your positive mood than getting the same total hours of sleep without interruptions. Those in the interrupted group had lower ratings of energy, vigilance, sympathy, friendliness, and agreeableness, the team reports.
This strong difference between the groups emerged after the second night and continued through the next day. This may suggest that these effects are cumulative, the researchers report, but more studies are needed to confirm this.
This hit to a positive mood may be explained by the lost opportunity to transition from one sleep stage to the next, the researchers suspect. Healthy people normally transition through five cycles of sleep every night, where stages 3 and 4 consist of slow-wave, or deep sleep.
“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,” study lead author Patrick Finan, an assistant professor of psychiatry and behavioural sciences at the Johns Hopkins University School of Medicine, said in a press release.
At least the short-sleepers had a few uninterrupted sleep cycles built in.
If you’re constantly being interrupted before stage 3, you’ll never get the opportunity to transition into the restorative deep sleep you need to feel refreshed in the morning. And even a little of that deep sleep is much better than none at all.
The study has several limitations, however. They used self-reported questionnaires, which are subject to bias, and mood was also only sampled once per day: in the evening. That can’t account for fluctuations that may have occurred throughout the day and night.
The researchers also can’t definitively say that disrupting the sleep cycle was what caused a reduction in positive mood, since they only indirectly manipulated slow-wave sleep by randomly forcing people to wake up. To prove causation, researchers would need to directly disrupt both slow-wave and REM sleep — the final of the five stages of sleep — and see how differently people fare.
Still, the study’s findings might help explain how people with insomnia, which is extremely common, can transition to depression without any pre-existing conditions for mood disturbances. An inability to feel positive emotions, which this study found is largely impacted by an interrupted sleep, is one of the core symptoms of depression, the researchers note.
The findings are also relevant to anyone who might be awoken frequently during the night, such as new parents, on-call health workers, or those who work in combat zones.
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