Those devices use a special chemical that absorbs light energy, and then releases it in a slow greenish or bluish glow. My el-cheapo alarm clock uses that stuff. Right after you turn off the lights, it glows brightly, but fades quickly as the energy it absorbed is given off as light. However, thinking about that, made me think about fireflies - that staple of summer nights that makes dark evenings sparkle. Scientists know that unlike other forms of light, the firefly's light, which is produced by a chemical reaction between adenosisine triphosphate, luciferin, oxygen, magnesium, and luciferase, in the firefly's abdomen, doesn't produce heat.
Nerve stimulations release a chemical inorganic known as pyrophosphate and light is produced.
My dad used to catch lightning bugs and hide from his sisters in the dark nights of the ranch where he was raised. Predictably, his sisters would run off screaming, and he'd roll on the floor laughing. It's a wonder he didn't roll on the floor retching, because birds and bats won't even eat lightning bugs, because they taste so bad, and the juice is very toxic. Nevertheless, he lived into his late 70s, so I guess it really didn't hurt him much - or maybe he didn't swallow. There's one type whose females flash in many different ways to attract other species males.