Yes, it is relatively common to experience mild hallucinations in the dark, especially when in a low sensory environment. These hallucinations are known as "hypnagogic" when they occur while falling asleep or "hypnopompic" when they happen upon waking up. They are considered a part of the normal sleep-wake transition and are not usually a cause for concern.
The brain's visual system can become more sensitive to detecting patterns and interpreting ambiguous stimuli in low light conditions. When you're in a dark room, your brain may start to generate images or perceive faint shapes and movements, even though they are not actually present. These hallucinations can range from simple flashes of light or colors to more complex visual scenes.
Similarly, when you wake up in the middle of the night or early morning, your brain may still be in a semi-dreaming state, and you might experience brief hallucinations or vivid images until you fully wake up and the effect wears off.
These types of hallucinations are usually short-lived and tend to disappear as soon as there is enough light or when you become more alert. They are different from hallucinations associated with mental health disorders or substance use, which are typically more persistent and distressing.
If you find these experiences unsettling or if you experience hallucinations under other circumstances, it's a good idea to discuss them with a healthcare professional to rule out any potential underlying issues. In most cases, though, mild and occasional hallucinations in the dark are a normal aspect of human perception and brain activity.