Vaughan Bell in The Atlantic:
There is a brief time, between waking and sleep, when reality begins to warp. Rigid conscious thought starts to dissolve into the gently lapping waves of early stage dreaming and the world becomes a little more hallucinatory, your thoughts a little more untethered. Known as the hypnagogic state, it has received only erratic attention from researchers over the years, but a recent series of studies have renewed interest in this twilight period, with the hope it can reveal something fundamental about consciousness itself.
Traditionally, the hypnagogic state has been studied as part of the sleep disorder narcolepsy, where the brain’s inability to separate waking life and dreaming can result in terrifying hallucinations. But it’s also part of the normal transition into sleep, beginning when our mind is first affected by drowsiness and ending when we finally lose consciousness. It is brief and often slips by unnoticed, but consistent careful attention to your inner experience after you bed down can reveal an unfolding mindscape of curious sounds, abstract scenery, and tumbling thoughts. This meandering cognitive state results from what Cambridge University researcher Valdas Noreika calls a “natural fragmentation of consciousness” and the idea that this can be tracked over the early minutes of sleep entry is the basis of recent hypnagogia research.