These studies have been done, but they're basically worthless: they look at relatively small modulations around an insane baseline, like switching between 24, 28, and 30 hours on.

The major issue (other than $$$) is thought to be hand-offs: information gets lost when one caregiver takes over for another. For example, you might notice that Mrs. Jones looks just a little bit paler than she did earlier, and take that into account when treating her, even subconsciously. This gets lost when a new doctor takes over, and allegedly, the increase in hand-off-related issues is not outweighed by the decrease in sleep-related ones. That said, I'm also not convinced that this has been aggressively optimised, or even tested over long-enough durations that people adapt to increased hand-off frequencies.