If you're completely out of shape I guess I wouldn't recommend setting your alarm at 5:15 AM to commence vigorous jumping jacks.

But the generalisation that therefore 5:15 am workouts are detrimental to health is ludicrous.

I haven't heard anything that suggests that. In fact, most medical professionals suggest a morning workout for increased energy and better sleep patterns.

I'm guessing any circadian rhythms would adjust with your new waking up/exercise schedule making it no different to your body to exercise that early.