A person reports a brief, vivid sensory experience with no real stimulus. Before revealing the answer, decide what distinguishes a hypnagogic hallucination from a hallucination occurring during ordinary wakefulness.

Ask specifically when the experience occurred. A hypnagogic hallucination differs from a hallucination occurring during ordinary wakefulness purely by its timing, which is the single defining feature: it occurs specifically in the transitional period while falling asleep (or, in its hypnopompic form, while waking up), is usually brief, lasting seconds, and does not typically recur once the person is fully awake or fully asleep. A hallucination occurring during ordinary wakefulness, by contrast, arises while the person is fully alert, coexisting with otherwise intact perception of the real environment, without any specific tie to the sleep-wake transition. Timing relative to sleep onset or waking, rather than any difference in the sensory quality of the experience itself, is the key distinguishing feature.