For some inexplicable reason, Sony Mobile removed the ‘double tap to wake' feature from its phones since 2017. The last Xperia phone to feature tap to wake was the Xperia XZ. We've never understood the reasoning behind its removal, particularly as it used to be a standard Xperia feature year after year – and most other phones these days.
While the feature has not returned in its most simplest form, the Xperia XZ3 does bring it back in a more convoluted way. If you delve into the "Ambient display" options and enable "Smart activation", then double tapping the display will show the clock and notifications, double tapping again will show the lock screen. So the method is slightly contrived, but it works. However, unfortunately, double tap to sleep remains off the cards (for now at least).
Xperia XZs and XA1 system dump now available
Xperia XZ announced for all Japanese carriers; launches in early November
Sony Xperia XZ1 announced with iterative improvements and Android Oreo
Xperia XZ Premium Dual has a hybrid SIM tray after all