What are sudden, short jerking movements occurring when the body begins to relax in bed?

Hypnic jerks. Jerking movements occurring as one is falling asleep are called hypnic jerks and are normal. Jerks during sleep may be periodic limb movements if they are repetitive, brief, and occur in series. If they cause poor sleep or daytime sleepiness, they can be treated, but they often occur in normal subjects. Jerks may occur with sleep apnea , parasomnias (abnormal sleep behavior), and seizures.
Sleepiness. Any cause of sleepiness can cause them, whether insufficient sleep, narcolepsy or sleep apnea. They are considered a sign of severe sleepiness.