When you are falling asleep, you may feel hypnic jerks, also known as
sleep beginnings, which are abrupt, involuntary contractions of your muscles.
These jerks happen during the transition from wakefulness to sleep,
which is referred to as hypnagogic (short for hypnic).
Hypnic jerks usually only affect one side of the body, like the left
arm or leg, and occur seemingly randomly as you fall asleep.