Well, there is no general answer to this question as it depends on the style of system that you have. While it may be safe to sleep with your portable Heating, Ventilation, plus A/C units, it could be dangerous to have the same temperature settings as while I was in the day plus sleep in those same temps. Most homeowners, I included, use smaller systems such as the oil gas furnace plus the space gas furnace for emergency heating or when the central heating fails. These smaller electric furnaces are equipped with a safety feature that enables the equipment to switch off in case of overheating. Therefore it is safe to sleep with these units on as long as you lower the sizzling settings. Other major units such as the geo heat pump plus the ductless Heating, Ventilation, plus A/C can cause the utility bills to shoot up when left on, overnight, plus on un-even temperatures set too high. Even if there are different energy saving help tips that one can employ. One of the factors that can influence the option to leave the heating plus a/c on the same day settings is the lack of a controller that allows you to adjust the un-even temperatures on your bed. A simple solution for this would be a wireless thermostat. At times when my equipment is fresh from a furnace/heater tune-up session, I enjoy living it on since the service is usually improved after the sessions. The decision to leave your equipment on will absolutely be influenced by when boiler upgrade was done. Heating, Ventilation, plus A/C techs suggest that it would be more lavish to leave a equipment that was installed a long time ago. I also gained the same recommendation when I visited the heating corp to purchase a current heating equipment last fall.