During heating season, or cooling season?

Day (presuming upstairs bedrooms, which are unoccupied) or night (same, occupied)?

Infants & elderly (who may need their bedrooms quite warm), or "healthy younger" folks, who might prefer chillier bedrooms & heavier blankets?

How open are the stairways, for heat to easily rise? Or to be easily blocked with a light curtain, to keep warm air downstairs?