Have another look at one of the best controlled studies to date (Nedeltcheva 2010) that investigated the effect of sleep on fat loss and muscle retention.
The overweight participants that slept a little more than 5 hours a night lost a lot of weight after 2 weeks. But 80% of that weight was muscle.
The sound sleepers that slept 7.5 hours a night lost the same amount of weight, but only 52% came from muscle. That still seems like a lot, but do note that they weren’t doing weight training to maintain their muscle.
As you can see, the sound sleepers actually had 8.5 hours to spend on sleeping, but they only slept 7 hours and 25 mins on average, which probably ‘was enough’ to wake up naturally and refreshed.