Daydream is very, very CPU-intensive, especially if using games, so will tend to run hot. When it first released, it would occasionally get hot enough that the phone would interrupt the experience with a warning about excessive heat. Shortly thereafter there were a few updates, since when it still gets hot, but not to the same 'degree,' and I've been able to successfully use it for watching full-length movies, etc. I would definitely discourage leaving it connected to a charger while in use, though, as the combined heat from charging and running in VR may prove to be too much for the phone to safely use for an extended period.