“What did you see?” I asked her, as she sat across from me, wide-eyed and obviously unnerved even thinking back about the episode.

“It was horrible,” she said, “just horrible. I was lying on my back, and I saw it in the doorway, a big, dark, shadowy figure, blocking the door to my bedroom. I tried to scream, and nothing came out. I tried to raise my hands, to shield my eyes, to shoo it away, but I was paralyzed. I couldn’t move!”

“That must have been very frightening for you,” I said.

“It was like my brain was wide awake, but my body was not responding at all. I could see everything, but I could not lift a finger.”

Have you ever experienced anything like this before? Perhaps there was a night when you were exhausted, heading for bed, only to find that as you drifted off, you saw an ominous shadow on the wall or felt a heavy presence sitting at the foot of your bed. This left you terrified, afraid that you could not get to sleep, or worse yet, that you might never wake up.

This experience is called a hypnagogic hallucination. According to the Cleveland Clinic, they are common and usually take the form of seeing shapes, flashes of light, or images and patterns. Up to 70 percent of people experience these types of hallucinations at least once. Hallucinations are basically false perceptions of things that you see, hear, feel, taste, or smell. They can seem real, so real that they scare you! About 86 percent of these hypnagogic hallucinations involve thinking that you are seeing a face, figure, or person. 25 to 44 percent of them can involve somatic experiences, such as feeling someone sitting on the edge of your bed, and only a quarter or less involve sounds or voices. While they can be a symptom in people who have narcolepsy, a sleep and wakefulness disorder, most happen in people who have not been diagnosed with that disorder.

Are you dreaming when you have a hypnagogic hallucination? Not exactly. When you wake from dreaming, you almost always realize that you were just dreaming and now you are awake, whereas in the latter case, it takes a little while to sort out what is real and what is not. For most people, these phenomena are entirely benign and harmless. In a small group of people, however, they can be a symptom of narcolepsy, excessive daytime sleepiness, insomnia, or a mental health disorder. Can you do anything to lessen your chance of having these sometimes-scary experiences? Get plenty of quality sleep, avoid alcohol just before bedtime, and follow a regular sleep schedule.

Now that you feel better and can drift off to dreamland, let me warn you of one other phenomenon related to hypnagogic hallucinations and sounds awfully similar. Hypnopompic hallucinations happen when you are waking up and in that twilight stage just before being fully awake, and they can be remarkably similar to what we discussed above. Rest assured that these events are almost always benign, harmless, not associated with any significant illness, and are nothing to be afraid of.

Sweet dreams.

Greg Smith is a psychiatrist.