The color of light, expressed in Kelvins (K). The sun is about 5800 Kelvins. This is called a "temperature," because the hotter fire gets, the more it changes from a warm yellow glow to white.

The first gadget from the company known for its mail-order mattresses, Glow has a warm LED light that activates when you flip it over, then gradually dims as you wind down. In the morning ...