Here's the segment from Sagan's Demon-Haunted World on sleep paralysis. I'm not seeing the PDF online anymore.
A common, although insufficiently well-known, psychological
syndrome rather like alien abduction is called sleep paralysis.
Many people experience it. It happens in that twilight world
between being fully awake and fully asleep. For a few minutes,
maybe longer, you're immobile and acutely anxious. You feel a
weight on your chest as if some being is sitting or lying there. Your
heartbeat is quick, your breathing laboured. You may experience
auditory or visual hallucinations of people, demons, ghosts,
animals or birds. In the right setting, the experience can have 'the
full force and impact of reality', according to Robert Baker, a
psychologist at the University of Kentucky. Sometimes there's a
marked sexual component to the hallucination. Baker argues that
these common sleep disturbances are behind many if not most of
the alien abduction accounts. (He and others suggest that there
are other classes of abduction claims as well, made by fantasyprone
individuals, say, or hoaxers.)
Similarly, the Harvard Mental Health Letter (September 1994)
comments, Sleep paralysis may last for several minutes, and is sometimes
accompanied by vivid dreamlike hallucinations that give rise
to stories about visitations from gods, spirits, and extraterrestrial
creatures.
We know from early work of the Canadian neurophysiologist
Wilder Penfield that electrical stimulation of certain regions of
the brain elicits full-blown hallucinations. People with temporal
lobe epilepsy - involving a cascade of naturally generated
electrical impulses in the part of the brain beneath the forehead
- experience a range of hallucinations almost indistinguishable
from reality: including the presence of one or more strange
beings, anxiety, floating through the air, sexual experiences,
and a sense of missing time. There is also what feels like
profound insight into the deepest questions and a need to
spread the word. A continuum of spontaneous temporal lobe
stimulation seems to stretch from people with serious epilepsy
to the most average among us. In at least one case reported by
another Canadian neuroscientist, Michael Persinger, administration
of the antiepileptic drug, carbamazepine, eliminated a
woman's recurring sense of experiencing the standard alien
abduction scenario. So such hallucinations, generated spontaneously,
or with chemical or experiential assists, may play a
role, perhaps a central role, in the UFO accounts.
But such a view is easy to burlesque: UFOs explained away as
'mass hallucinations'. Everyone knows there's no such thing as a
shared hallucination. Right?
As the possibility of extraterrestrial life began to be widely
popularized - especially around the turn of the last century by
Percival Lowell with his Martian canals - people began to
report contact with aliens, mainly Martians. The psychologist
Theodore Flournoy's 1901 book, From India to the Planet Mars,
describes a French-speaking medium who in a trance state drew
pictures of the Martians (they look just like us) and presented
their alphabet and language (remarkably like French). The
psychiatrist Carl Jung in his 1902 doctoral dissertation
described a young Swiss woman who was agitated to discover,
sitting across from her on the train, a 'star-dweller' from Mars.
Martians are innocent of science, philosophy and souls, she was
told, but have advanced technology. 'Flying machines have
long been in existence on Mars; the whole of Mars is covered
with canals' and so on. Charles Fort, a collector of anomalous
reports who died in 1932, wrote, 'Perhaps there are inhabitants
of Mars, who are secretly sending reports upon the ways of this
world to their governments.' In the 1950s there was a book by
Gerald Heard that revealed the saucer occupants to be intelligent
Martian bees. Who else could survive the fantastic right
angle turns reported for UFOs?
But after the canals were shown to be illusory by Mariner 9 in
1971, and after no compelling evidence even for microbes was
found on Mars by Vikings 1 and 2 in 1976, popular enthusiasm for
the Lowellian Mars waned and we heard little about visiting
Martians. Aliens were then reported to come from somewhere
else. Why? Why no more Martians? And after the surface of
Venus was found to be hot enough to melt lead, there were no
more visiting Venusians. Does some part of these stories adjust to
the current canons of belief? What does that imply about their
origin?
There's no doubt that humans commonly hallucinate. There's
considerable doubt about whether extraterrestrials exist, frequent
our planet, or abduct and molest us. We might argue about
details, but the one category of explanation is surely much better
supported than the other. The main reservation you might then
have is: why do so many people today report this particular set of
hallucinations? Why sombre little beings, and flying saucers, and
sexual experimentation?