HR, calorie count, and saving battery life

I turned off the wrist-based HR monitor to save on battery life. That causes the watch to stop counting calories during activity. Is there a way to have it automatically turn on the HR monitor during an activity, so that I can get a calorie count, and otherwise stay off?

Note: I see the option to only broadcast HR during activity, but that doesn't seem to solve the issue. (I think that feature assumes you are still measuring HR all day.)

You would have to turn the OHR back on manually.

But the OHR on the F5 is pretty energy-efficient (unlike other earlier OHR devices) in 24/7 mode. Even with OHR running, battery life is a couple of weeks (ie. a few percent a day), so it really is a pretty minor drain.

If you want to save even those few percent of battery, just take the watch off at night - the OHR LEDs deactivate after a few seconds. With the OHR turned off manually, you wouldn't be getting sleep data from the watch anyway, as it combines movement with HR data to judge sleep.