hypnagogic

Hypnagogic describes phenomena associated with the hypnagogic state, the transitional phase between wakefulness and sleep that occurs as a person falls asleep. The term derives from Greek hypnos, sleep, and agogos, leading or guiding.

During this period, sensory impressions can intrude into wakefulness, including vivid visual imagery, geometric patterns, sounds, or tactile sensations. Hypnagogic imagery is often dreamlike or surreal, while hypnagogic hallucinations are more vivid and may be frightening. A common companion is the hypnic jerk, a brief, involuntary muscle contraction as sleep onset occurs.

Physiologically, these phenomena accompany the onset of sleep, with shifts from wakeful brain activity toward sleep rhythms (often theta) and changes in thalamo-cortical processing. They tend to occur before full sleep and typically fade as deeper sleep takes hold. They are distinct from hypnopompic phenomena, which occur during the transition from sleep to wakefulness.

Most people experience hypnagogic phenomena at least occasionally, and they are generally considered normal. They can be more frequent with sleep deprivation, stress, or irregular sleep schedules and are sometimes reported in narcolepsy. Some researchers note a possible link to creativity, given the unusual imagery that can arise at sleep onset.