Well, there is no general answer to this question as it depends on the type of system that you have. While it may be safe to sleep with your portable HVAC units, it could be dangerous to have the same temperature settings as during the day and sleep in those same temps. Most homeowners, I included, use smaller systems such as the oil heater and the space heater for emergency heating or when the central heating fails. These smaller electric heating systems are equipped with a safety feature that enables the unit to switch off in case of overheating. Therefore it is safe to sleep with these units on as long as you lower the warm settings. Other major units such as the geo heat pump and the ductless HVAC can cause the utility bills to shoot up when left on, overnight, and on 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 choice to leave the heating and air conditioning on the same day settings is the lack of a controller that allows you to adjust the temperatures on your bed. A simple solution for this would be a wireless thermostat. At times when my unit is fresh from a furnace/heater tune-up session, I like living it on since the service is usually improved after the sessions. The decision to leave your unit on will definitely be influenced by when boiler installation was done. HVAC techs advise that it would be more costly to leave a unit that was installed a long time ago. I also received the same recommendation when I visited the heating corp to purchase a new heating unit last fall.