Would this be advisable/possible for someone who does moderate to heavy physical activity (lifting heavy at the gym, dancing) almost daily?

Not advisable. You will recover more slowly and be more at risk of overtraining.

Based on my experience and that of people I've spoken to, I would say dancing is fine, lifting is not. I can swim almost infinitely, do martial arts, and even climb moderately with no effect on my sleep; but those things are not tearing muscle generally. Things like lifting and heavy climbing, which tear down / build muscle -- and insanely calorie-intense things like whole days of skin-diving; I learned that one the hard way -- will necessitate extra sleep, often even after adaptation (just like they do when you're monophasic); but during adaptation, they could really screw you up by preventing you from sticking to the schedule. Stay in shape by keeping your cardio and fitness activities, but cut out the serious limits-pushing training for those few weeks, and plan to take some extra naps or a longer core when you do them afterwards, and polyphasic sleep and athletics seem to get along just fine.