People often experience muscle twitching just as they are falling asleep, which may wake them up and cause a fright. These muscle spasms are referred to as hypnagogic jerks, but may also be called hypnic jerks or sleep starts.
Live Science reported in 2013 that studies have shown that 60 to 70 percent of people experience sleep starts that can include dreams, hallucinations (visual and audible) and the feeling of falling.
Dr. Carl Bazil, director of the Sleep Disorders Center at New York–Presbyterian/Columbia University Medical Center, recently told New York Magazine that science doesn't know exactly what causes sleep starts, but the condition may happen because part of the brain wants to sleep, but another part wants to stay awake.
“One of the things that happens as you fall asleep is your muscles relax, but the awake part may still be stimulating enough that it will temporarily overreact and you get this jerk of muscle activity,” Bazil stated.
“It doesn’t usually mean anything, so there hasn’t been a huge amount of resources devoted to figuring it out,” Bazil added.
However, medical professionals do know medications that cause stimulation, tobacco and coffee, can make sleep starts more difficult.
"The most common culprit by far would be caffeine,” Bazil said. “If it’s bothersome, the first thing I tell people is to cut back on the caffeine, especially late in the day.”