Hey there!
Thanks for asking the community!
I understand that you're curious about iPad discharging while sleeping (display is off, but the iPad is on).
iPadOS does a series of things in the background, ranging from system tasks, to 3rd party app refreshes. All of this uses energy on the battery, which in time, can cause it to go dead.
This link from Apple's support webpage has some useful information regarding the kind of batteries used in Apple devices (lithium-ion), and some more about their behavior: Charge the iPad battery - Apple Support
However, if your iPad battery still seems to run down fast, you can contact Apple Support for a diagnostic! An AppleCare Advisor has the ability to check the health of the battery, and can let you know if it's recommended to service your iPad. You can call AppleCare at 1 (800)APL-CARE (1-800-275-2273), or on their website at https://getsupport.apple.com/
I hope this helped!