Back when the Aerial screen saver was first released there was a bug that would prevent Macs from going to sleep. The developer fixed that bug a while ago. If this is still an issue for anyone they should try download the latest version of the screen saver from the following link.
For those that are curious. The screen saver was originally written to use AVPlayerView which by design keeps Macs awake when its playing a video. Aerial's developer rewrote the screen saver to use AVPlayerLayer instead, which does allow Macs to sleep when they are playing video but otherwise idle.