Lots of people have been having this issue since mid-winter. I gave up trying to resolve it after following the posts for a few weeks. Came back and now see there is a solution. I deleted the app and reinstalled; now my sleep stats appear to be readily accessible. Why don't you push out a notification to everyone to let them know the solution? Lots of users don't even know these help boards exist. They just grin and bear the problems and grouse to their friends about their issues with Fitbit.
I am on my third fitbit in five years. Keep coming back because they're relatively cheap for what you get. But all the issues with the Charge 4/iPhone are making me rethink my next purchase: sleep stats inaccessible for months, reminders to move don't work if you're sitting at a computer (the watch thinks you're asleep), unwanted notifications for steps and zone minutes at 2x the goal (even though notifications are turned off), Bluetooth connection is extremely spotty and requires rebooting the watch from a screen that takes 11 swipes/touches to reach.
