That sudden jolt is most likely a hypnic jerk, also called a sleep start. It is a brief, involuntary muscle movement that usually happens as someone transitions from wakefulness into light sleep. The whole body may jump, or the movement may affect only an arm or leg. Some people also experience a sensation of falling, a flash of light, or a dreamlike feeling at the same moment.
Although the exact cause is not fully understood, hypnic jerks are extremely common and usually harmless. They may occur because the nervous system is changing activity as the muscles relax during sleep onset. The brain may briefly interpret that relaxation as falling and trigger a quick muscular response.
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