A voice says: “Close your left hand. Don’t ask yourself whether you’re asleep. Think about trees.”

I’m lying in bed. A sleep mask covers my eyes. A tangle of wires covers my left hand. At the tip of my ring finger, a sensor measures my heart rate. A flexible length of plastic embedded with circuits stretches from my palm to the top of my middle finger. This will record the hypnic jerks and spastic opening-hand motions that signal my entry into hypnagogia, the first stage of sleep, where thoughts slip free of conscious control.

There’s a laptop on the…