On your iPhone/iPad:
Just open Pillow on your iPhone/iPad and tap the sleep diagram below the rings. This will bring forth the last sleep analysis report. Scroll down to view a detailed breakdown of your sleep session or swipe left/right on the screen to access previous sessions/return to current date.
On your Apple Watch:
Open Pillow on your Apple Watch and scroll down with your finger or with the Apple Watch digital crown to find all the details about your latest sleep session, such as Quality, Duration and more.