Ever climb into bed, get all warm and cosy and start to drift off to sleep, only to feel a sudden jerk that wakes you up? It might be a sign that you're exhausted.
"These can be referred to as sleep starts or hypnic jerks," University of Wollongong Australia associate professor Christopher Magee told The Huffington Post Australia.
"They occur when there is a sudden jerk of part, or all of the body, that occurs during the transition between wake and sleep. These can occasionally awake an individual and/or their bed partner."
The good news is that a hypnic jerk is not a sign of anything untoward and they occur in perfectly healthy people.
"The precise causes and underlying mechanisms remain unclear, however in the vast majority of cases they represent a normal event and can be experienced by healthy individuals," Magee said.
Though it is reported that you're more likely to experience a hypnic jerk if you're particularly tired or run down, so you might like to look at your routine and lifestyle.
"The intensity of these jerks can be influenced by factors such as fatigue, sleep deprivation, stress, and consumption of alcohol. Healthy sleep habits (e.g., regular bed and wake times, healthy diet, limiting screen time) may help to minimise these sleep starts."
Though you may feel that hypnic jerks occur when you dream that you're running, falling or being thrown a ball, dreaming and jerks actually occur at different stages of sleep.
"Hypnic jerks occur during the transition into stage one sleep, whereas dreaming tends to occur during a stage of sleep called REM (rapid eye movement sleep). Some dreams can be quite vivid, so it is plausible that individuals ‘act out’ in response to a dream by moving body limbs. However, sleep and dream are incredibly complex, and we still do not have a clear understanding of the causes and implications of these processes," Magee said.
One theory is that the hypnic jerk could have evolutionary significance, as it is belived that when humans slept in trees the hypnic jerk would prevent us from falling out, however this is not proven.
If you have trouble nodding off, Magee suggests assessing your pre-bed routine to ensure you're setting yourself up for the best night's sleep possible, free of sleep starts.
"Factors such as stress, caffeine, alcohol consumption, uncomfortable bedroom environments, screen time (e.g., television, computer usage, mobile devices), lighting, environmental noise, jet lag, and shift work can make it difficult to fall asleep."
"Worrying excessively about difficulties falling asleep could worsen any difficulties falling asleep. For most individuals, it is important not to become preoccupied with these issues and instead try to focus on healthy sleep routines and lifestyles," Magee said.