What is sleep paralysis, and is it rare?
—Mark Fischetti, Lenox, Mass., editor of Scientific American Earth 3.0

Psychologist Christopher French of Goldsmiths College in London explains:
Attacks by demons, ghostly visitations and alien abductions: some people are certain they have experienced such paranormal events. In reality, many of these victims probably had an episode of sleep paralysis.

Sleep paralysis, a momentary inability to move one’s limbs, trunk and head despite being fully conscious, may occur when someone is either drifting off or, more rarely, waking up. During rapid-eye-movement (REM) sleep, the muscles of the body are paralyzed, presumably to prevent the dreamer from physically acting out the dream. Researchers are not sure why this normal paralysis happens during consciousness for victims of sleep paralysis, but psychophysiological studies have confirmed that attacks are particularly likely to occur if the person enters REM sleep quickly after hitting the pillow, bypassing the stages of non-REM sleep that usually happen first.

Other factors that make sleep paralysis more likely to occur include drift­­­ing off while lying on the back, feeling stressed or experiencing a disruption in normal sleep patterns, such as from shift work, jet lag, caffeine or alcohol.

Although sleep paralysis is a symptom of narcolepsy, it is also common in healthy people. Surveys from different countries show a wide range of estimates: 20 to 60 percent of the normal adult population has experienced sleep paralysis at least once. Around 5 percent of the population has experienced one or more of other disturbing symptoms associated with the disorder. The most common effects include visual hallucinations, such as shadows and light or a human or animal figure in the room, and auditory hallucinations, such as hearing voices or footsteps. A person often also feels pressure on his or her chest and has difficulty breathing.

The reason sleep paralysis may explain tales of ghosts and aliens is the strong sense of a presence, usually harmful, that victims commonly feel during an attack. They also report unusual kinesthetic sensations, such as feelings of being dragged out of bed, vibrating, flying or falling. These episodes can sometimes lead to full-blown out-of-body experiences. Sleep paralysis may be frightening, but it is never dangerous, and thankfully, episodes usually last only a few seconds.

Why am I sometimes awakened in the middle of the night by explosions going off in my head?
—Jade Peifer, Cypress, Fla.

Randolph W. Evans, professor of neurology at the Baylor College of Medicine, responds:
There may be several reasons why you’re experiencing these explosions erupting in your head. Perhaps you’re in love, as the lyrics to Atreyu’s “When Two Are One” suggest:

Bang!
Explosions in my head
that just won’t quit.
A train has crashed into the
wall around my heart ...

Alternatively (and more likely), you have an uncommon sleep disorder, which in 1988 British neurologist John Pearce named “exploding head syndrome.”

During an episode, a person feels a loud bang coming from inside his or her own head, often described as an explosion, a roar or waves crashing against rocks. Eruptions generally occur while people are falling asleep and less frequently when they are waking up. The explosions vary in frequency and happen most often in healthy individuals older than 50. In 10 percent of cases, people perceive a flash of light, and about 5 percent of patients report the sensation that they have stopped breathing and must make a deliberate effort to breathe again. Sufferers may be afraid or anxious in the aftermath of an attack.