While falling asleep, the human body endures cycles of muscle contractions that are often experienced as muscle twitches. These twitches are referred to as hypnic jerks or "sleep starts."
As a person drifts off to sleep, he or she may experience muscle twitches ranging from mild to intense. These involuntary muscle spasms normally occur during hypnagogia, which is the stage in which the body is falling asleep. Usually the twitches are brief, most only lasting a few seconds or so. Sometimes, however, they can be more intense. These twitches are referred to as "exploding head syndrome" and, while the name is scary, the syndrome is not dangerous and is actually quite rare. The people who suffer from this syndrome experience a sensation similar to thunder inside the head, or the sound of cymbals crashing.
Even though muscle twitches are common and usually harmless, sometimes they can interfere with getting a good night's sleep. Some experts in the scientific community believe that the twitches may be associated with stress or anxiety, or even with an irregular sleep schedule. If the twitches become exceedingly disruptive, one should consult a doctor specializing in sleep medicine. He or she can also try to incorporate improvements to the sleep routine, such as cutting back on caffeine and going to bed and waking up at the same time each day.