But why the automatic sleep?

The device goes to sleep automatically when the music is not strong enough, or there is more music in the song (just the singer talking for example). I can let you imagine what that's painful to see the mess for songs where the music goes down very gradually for a long time by horrible deafening hiss. Deafening, because you have to push the volume of his radio above normal for an equivalent with the transmitter (this being, having already used other FM transmitters, it is not worse than the others). So to summarize, if you listen to other things than music or albums passage interspersed with a low sound or lyrics without music, avoid this.
Last thing, for some reason, but even for "normal" music, impossible to listen to a stream from Youtube, the device switches automatically to standby after one minute.