For safety reasons, sleeping with electric heaters on is not good. If you are sleeping in a room with a portable heater, make sure that you turn it off before sleeping. If you talk about the best portable car heater sitting on your car dash, you need to take some precautions. First, you must provide a nice crack in the window for ventilation. Secondly, choose portable heaters which have ODS, safety auto-shutoff, and tip-over shut-off. Sleep with a portable heater on if only it is very necessary as carbon monoxide pollution in an enclosed space can cause health issues like dizziness, headache, feeling nauseous, etc.