Jolting awake when drifting off to sleep – what are hypnic jerks?

People who often jolt awake just as they're about to drift off to sleep have poorer sleep quality and higher levels of anxiety, according to a new study.

Research from the Paris Lodron University Salzburg in Austria, published this week in PLOS Mental Health, surveyed 194 adults in Austria and Germany, comparing those who startle awake – also known as "sleep starts" or hypnic jerks – at least weekly with those who never or only rarely do.

Sleep starts are sudden jolts that occur in the moments between wakefulness and sleep. Most often felt as an abrupt muscle jerk or twitch, the sensation can sometimes be strong enough to jolt a person fully awake. It can also involve a sudden sensation of falling or loss of balance; an electric shock; tingling or numbness; flashes of light; or brief, dream-like imagery. These experiences are frequently accompanied by a rush of emotion – such as alarm, confusion, fear, or panic – and physical signs of being startled, such as a racing heart or rapid breathing. Most people fall back asleep soon afterwards.

The study's participants answered an online questionnaire about their sleep quality, insomnia severity, daytime sleepiness, dysfunctional beliefs about sleep – such as how much sleep they "should" be getting – and symptoms of anxiety and depression.

Ninety-six of the participants experienced abrupt awakenings and at least one other symptom weekly, while 98 others were used as a control group.

Those who experienced the phenomenon reported poorer sleep quality, greater insomnia severity, more unhelpful sleep-related beliefs, and higher anxiety and depressive symptoms than those in the control group, according to the researchers.

The frequency with which people experienced sleep starts were also linked to sleep quality, sleep beliefs, and anxiety, researchers said – but not depression. Those factors, as well as age and gender, were associated with one-quarter of the differences between people in sleep start frequency (26.9%).

Participants also reported more frequent sleep starts during times of psychological stress and emotional strain, whereas sleep deprivation had less of an influence on the frequency of sleep starts.

The authors said causality couldn't be inferred from the participants' self-reported data but the results highlighted "potentially relevant links between sleep starts and sleep-related cognitive-emotional processes".

One of the authors of the study, Tanja Grünberger, said sleep starts were "much more than a simple muscle twitch when falling asleep".

"Participants described not only jerks, but also sensations of falling, a racing heart, flashes of light or loud bangs, and strong feelings of alarm or fear – and those who reported sleep starts at least once a week also reported poorer sleep quality, more unhelpful beliefs about sleep and more anxiety symptoms."

She added an interesting finding was the close link between sleep starts and “how people think and feel about sleep”.

"The more strongly participants held unhelpful beliefs about sleep and the more anxious they were, the more often they reported sleep starts – and in the moment itself, many described a strong pressure to finally fall asleep. Our data cannot show what causes what, but this pattern resembles what we know from insomnia."

Grünberger added that because there was no established questionnaire for sleep starts, researchers had developed their own based on clinical literature.

"This allowed us to capture a broad range of experiences – but it also means that our findings now need to be confirmed with validated instruments and objective sleep recordings."

Senior author Manuel Schabus said while for most, suddenly jolting awake was "a harmless feature of falling asleep”, for a subgroup it was intertwined with distress and taking a longer time falling asleep.

"Whether approaches that reduce arousal and sleep-related worry also help with distressing sleep starts has not yet been tested – that is an important next step," he said.