The Energy Bucket
Glow-in-the-dark stickers have special chemicals called phosphors. When your bedroom light is on, these phosphors act like tiny buckets that catch 'light rain' (energy) and store it up. The atoms inside the sticker get 'excited' and vibrate with all that extra energy.
The Slow Leak
When you turn the lights off, the 'buckets' start to leak. Unlike a mirror that bounces light back immediately, phosphors release their energy very slowly. That soft green glow is just the daytime light energy slowly leaking back out into the dark room.
Charging the Stars
The brighter the light you use to 'charge' your stickers, the more energy the phosphors can store. This is why a sticker held right up to a lamp will glow much brighter than one on the far side of the room. Ultraviolet (UV) light, like the kind from the sun, is especially good at filling up these 'light buckets' quickly.