Sleep paralysis is a phenomenon in which a person either during falling asleep (Hypnagogia) or awakening (Hypnopompic), temporarily experiences an inability to move, speak, or react. It is a transitional state between wakefulness and sleep, characterized by muscle atonia (muscle weakness). It is often accompanied by terrifying hallucinations (such as an intruder in the room) to which one is unable to react due to paralysis, and physical experiences (such as strong current running through the upper body). One hypothesis is that it results from disrupted REM sleep, which normally induces complete muscle atonia to prevent sleepers from acting out their dreams. Sleep paralysis has been linked to disorders such as narcolepsy, migraines, anxiety disorders, and obstructive sleep apnea; however, it can also occur in isolation.