Q: Why do I jerk before falling asleep?

These involuntary muscle spasms are actually very common—and harmless. Known as hypnic jerks or "sleep starts," the spasms may make you feel as if you're falling or have suddenly been "shocked" awake. Most experts speculate that these jerks happen as a result of your nervous system relaxing as you slip into sleep mode; it's also possible that neurons misfire because the brain misperceives the muscle relaxation as actual falling.

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Hypnic jerks may occur more often when you're stressed-out or sleep-deprived, or if you work out or have caffeine close to bedtime. If they are regularly disturbing your ability to fall or stay asleep, though, see a sleep specialist: They may be linked to an underlying disorder, like restless legs syndrome or sleep apnea.

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Health’s medical editor, Roshini Rajapaksa, MD, is assistant professor of medicine at the NYU School of Medicine.