I have the same problem lol, the app told me I have unusually long heavy sleeps. Can't really sleep as much as I want during the week but you bet I am sleeping insane hours on the weekend.
Their privacy notice states they keep the information on their servers. It can be shared but I have my phone set to where it doesn't share my data to other sources or track data from other apps. I've worked data privacy for 10 years, on a scale from off the grid prepper to Facebook Cambridge Analytica Scandal.....it falls pretty low. That said, as a general note make sure the privacy settings on your phone are updated to your liking. I loathe advertisements and so much more when they're targeted.