Hypnagogic hallucinations

"Sometimes, shortly after I've fallen asleep, I suddenly wake up and see a squirrel sitting at the foot of my bed," a patient told me. "But it's not a dream. I really do see it. Except it's not there."

"How long does it take you to figure out it's not really there?"

"About ten seconds. I jump out of bed - screaming - and turn on the light and it's gone."

"How often does this happen?"

"Maybe every six months or so."

The patient was a twenty-year-old woman with an unremarkable medical history, and no other psychiatric symptoms.

She was describing hypnagogic hallucinations, vivid and often frightening experiences that occur while falling asleep. They are usually visual or auditory. I had another patient who would wake up to her bedroom door slamming shut, when in fact it was still open. The hallucination typically lasts for seconds to minutes.

Hypnopompic hallucinations is the same phenomenon, but upon waking.

Hypnagogic  hallucinations are common among healthy children, may occur in healthy adults, and can be a symptom of narcolepsy.

I've experienced these hallucinations half a dozen times in my life. I've seen a massive spider on the ceiling, and a man standing next to the bed. Both were terrifying, but I daresay a squirrel trying to get in under the covers would be worse.