There are several elements that make a story a really good bedtime story. Firstly, the theme should be positive and reassuring. Stories about friendship, love, or a safe home environment work well. Secondly, the use of repetition can be very effective. This can be in the form of a repeated phrase or a recurring event in the story. It gives a sense of familiarity and comfort. The length of the story also matters. It should be long enough to be engaging but not so long that it over - stimulates the listener. A story that takes about 10 - 15 minutes to read is usually a good length for bedtime.
A simple plot makes a really good bedtime story. It should not be too complicated so that the listener can easily follow along as they are getting sleepy. For example, a story about a little bunny going to sleep in its burrow is easy to understand.