A hypnagogic jerk is an involuntary muscle spasm that occurs as a person is drifting off to sleep.
The phenomenon is so named in reference to the hypnogogic state — the transitional period between wakefulness and sleep. Hypnagogic jerks are also commonly known as hypnic jerks or sleep starts.
The muscle spasms may occur spontaneously or may be induced by sound, light or other external stimuli. Some people report hypnagogic jerks accompanied by hallucinations, dreams, the sensation of falling, or bright lights or loud noises coming from inside the head.
Sleep starts are quite common, with some research suggesting that 60 to 70 percent of people experience them. Many individuals may be visited by nightly hypnic jerks without even knowing it, as the twitches often go unremembered, particularly if they don't cause a person to wake up.
Some scientists believe certain factors, such as stress, anxiety, fatigue, caffeine and sleep deprivation, may increase the frequency or severity of hypnagogic jerks, but conclusive research is lacking on the subject.
Researchers are also unsure why hypnic jerks occur, but a few theories exist.
One hypothesis says that hypnagogic jerks are a natural part of the body's transition from alertness to sleep, and occur when nerves "misfire" during the process.
Another popular idea takes a more evolutionary approach to hypnic jerks, explaining that the spasms are an ancient primate reflex to the relaxation of muscles during the onset of sleep — the brain essentially misinterprets the relaxation as a sign that the sleeping primate is falling out of a tree, and causes the muscles to quickly react.