Answer: Hypnagogia

Though it may seem so at first glance, sleep isn’t a simple binary affair of the period we spend awake and the period we spend asleep. At the end of the day, when you lay your head on a pillow and prepare to drift off to sleep, there is a distinct period, a transitional state, called “hypnagogia”.

During this period where the body is transitioning between complete wakefulness and sleep, there is a significantly increased chance of experiencing a wide array of sensory experiences. Some people see rainbow-like light shows (appearing as light across their closed eyelids, but actually just a phenomenon known as “phosphenes” where your visual nerves are stimulated in the absence of actual light). People who play a lot of video games or perform repetitive actions at work may experience the “Tetris effect”, wherein scenes from the video game or their workplace play out as they drift off to sleep. Other sensations include hearing things—an experience which ranges from something as simple and calming as hearing a fan-like white noise to, for those with “exploding head syndrome”, hearing incredibly startling noises—smelling things, feeling hot or cold, or any number of sensations the human body can perceive.

The state is also said to be a highly creative one and many people prize the period of semi-lucidity that occurs between wakefulness and sleep. There is an apocryphal tale about Thomas Edison that states he would often fall asleep holding steel ball bearings so that as he finally drifted off into deeper sleep and relaxed his hand, the clatter of the dropped bearings would wake him and he would record all his ideas. But Edison is hardly alone in giving credit to the insights the twilight of the day provides; Nikola Tesla, Isaac Newton, Beethoven, Salvador Dali, and other prominent inventors and artists over the years all stated that the period was a highly productive and valuable one.