It's not exactly like you're chopping firewood for 12 straight hours. But it is still physically demanding. With the rotating from nights to days and back, there is a serious disruption in the sleep cycle that has to be managed. Some people simply cannot manage it. Others have no trouble at all.
The key to staying awake on a night shift is to stay active. Keeping the heart rate up is necessary to avoid falling off at the controls.
Flipping is really not as bad as it may seem if your body and brain can tolerate it. Being inside a windowless building actually helps, since you can't tell night from day by the sunlight level. Night shift workers need to sleep during the day, so an environment free of light and noise is absolutely essential. The longer "weekends" (which may happen at times other than Saturday and Sunday) are meant to facilitate the change. If you don't treat them like mini-vacations, you can gradually adjust your sleep times.
As far as the OT question, it depends on the local contract and the schedule. "Built-in-overtime" violates the labor laws for non-exempt employees. You have to pay hourly workers for each hour, and you have to pay OT for all hours over 40 in a week. Even for salaried employees, scheduled hours wouldn't be considered built-in. But the rules for salaried people are different, and averaging is not necessarily a violation for them. Since their overtime doesn't have to be paid at time-and-a-half, they can simply base their salary on 48 hours (or whatever the average is) as opposed to 40.
Still, you have to refer to the local contract to see what any given plant will do in this case.