Counter anecdote. Night owl here. I was in the army. Forced wake up with no way to stay in bed at 6am or earlier, lights out midnight or earlier (in general - every couple of weeks we would stay up later for something or skip sleep altogether.

Most of my mates would be asleep 3 seconds before hitting the pillow; the other self-proclaimed night owls would take 15 minutes to fall asleep. I rarely fell asleep before 3 am.

After about 4 weeks, I exhausted whatever reserves there were, and started falling asleep at every opportunity during the day (good or not, sometimes while standing up, often instead of eating). After another couple of months, I was able to function with a couple of 15 minute power naps.

This lasted for 8 month until I managed to get a different assignment. Food wasn't according to your plan, but otherwise, I was forced into larking for a reasonably long time. My eyesight deteriorated, and health otherwise declined - but I remained owl throughout