[Prestidigitweeze]

I've learned that playing with my Galaxy S4, or dinking around on my laptop, will keep me awake for hours. Reading on my Aura HD, however, will put me out before you can say groggy stalactites.

It might be partly due to the kind of lighting used by the Aura, but I think it's also due to this:

The passivity of reading versus the active engagement of tablets (web surfing, texting, etc.).

When you read a work of fiction, you're the projectionist, the sound system, the CGI team, the vintage special effects crew, the cinematographer and the actors; you're effectively performing a work of fiction in your head. But in another sense, you're a child in the back seat of a truck being lulled by your Uncle Goren's story about his boyhood: It can put you out instantly. Spoken daydreams can hit you like verbal tranquilizer darts.