| Last updated
That sudden jolt is called a hypnic jerk and it is totally normal and extremely common. Dr. William Kohler, medical director of the Florida Sleep Institute and director of the paediatric sleep services at Florida Hospital, Tampa said as many as 70 percent of people experience hypnic jerks.
Speaking to NBC News, James K. Walsh, executive director and senior scientist at St. Luke's Sleep Medicine and Research Center said: "A hypnic jerk or sleep starts are a perfectly normal occurrence that is almost universal.
He added: "It involves a total body experience where your muscle contracts therefore your limbs jerk or your body twitches. They generally occur during the transition between wakefulness and sleep."
The body enters Stage 1 sleep as it begins to nod off, this is followed by Stages 2 and 3, then comes rapid eye movement (REM) sleep. These stages last around 90 to 120 minutes and the cycle continues four or five times a night.
Hypnic jerks usually occur between wakefulness and Stage 1 sleep.
Also known as a hypnagogic jerk, a hypnic jerk is basically involuntary muscle spasm. They tend to be very brief and last half a second or less.
Because hypnic jerks are not considered dangerous, there has not been lots of research into them which means they are not fully understood.
Rafael Pelayo, a sleep specialist at the Stanford Sleep Medicine Center, sat down for an interview with Mic about hypnic jerks.
According to Pelayo, as you transition from wakefulness to sleep, you might swing your arms or kick your legs involuntarily. If you're sitting upright and start dozing, you might snap your head back.
Despite them not being totally understood, hypnic jerks seem to occur when you're sleep deprived, but force your body to continue trudging along, like when you're trying to stay awake during a work meeting.
When you are sleep deprived, the body transitions from Stage 1 straight into REM, during which vivid dreams occur.
Pelayo explains that when our bodies are forced to carry on engaging in some activity, certain parts of the nervous system are asleep while other parts are awake. The nervous system's control of our muscle movements during sleep is different than it is during wakefulness. This explains why our limbs twitch.
Chosen for YouChosen for You
Most Read StoriesMost Read