Spotify supports sleep alarms on iOS and Android. There are 3 methods.
1.For iOS and Android users, free or paid users for Spotify: the best method I think is to convert Spotify to MP3 with a Spotify converter. After the conversion, just add the song to your Clock app.
2.Spotify Premium users on iOS: You can use Music Alarm Clock for Spotify, which is a paid apple.
3.For Android users, you can use Google Clock app.
More details go to: how to set Spotify as alarm