This app was mentioned in 2 comments, with an average of 2.50 upvotes
To imitate the stock launcher's double-tap to sleep in Nova, you can install this app (or similar) and set Nova's double-tap gesture to open it.
Oké I did, ;P I used this app,
https://play.google.com/store/apps/details?id=ndt.mobile.android.turnoffscreen