According to Barnes & Noble, the Nook Simple Touch with GlowLight provides "about a month" of reading time, compared to two months with the light off.
The centerpiece of the Nook SimpleTouch with GlowLight is, naturally, that illuminated screen. The reader uses a patent-pending technology Barnes & Noble built in-house, which shines a light from over the top of the screen. The illumination is triggered by holding the "n" button down for two seconds. You can adjust the brightness by clicking the button again and sliding the dial on a Quick Settings screen. Half brightness should be enough to keep you reading in a dark room. You can also turn the light off here or just hold down the "n" button again.
The light is actually quite well distributed on the display. You'll notice a bit of a gradient at the top, near the light source, but after reading for a bit, odds are you'll completely forget about it. We used it for a bit in a darkened room, and it's really a pleasant reading experience, staying true to most of the benefits of e-ink. We asked Barnes & Noble reps how the resulting eye strain compares to a back-illuminated device like the iPad, and while they refused to say anything conclusive, they conceded it should be "equal or less" to what you'd experience on such a device.