How long you can leave the AC on all night is indefinitely, as central AC is designed to run continuously 24 hours per day for months if needed, with no harm to unit, as long as filter clean and coils not frozen. Central AC like Trane XR16 or Lennox ML14XC1 is designed to cycle on and off automatically to maintain set temperature, running 15 to 20 minutes per cycle, then off for 10 to 15 minutes, so leaving on all night means it will run intermittently, not continuously, unless undersized or very hot night over 85 degrees, in which case it may run continuously for hours which is normal on extreme nights. There is no time limit that causes damage, AC can run all night every night for entire summer. For window AC and portable AC, they can also run all night but may need to be set to energy saver mode to avoid freezing evaporator coil. Concerns about leaving AC on all night include electricity cost of 1 to 3 dollars per night for central AC at 3,000 watts running 8 hours at 15 cents per kWh, dry air causing throat dryness, and noise at 70 decibels for single stage. To run all night efficiently, set thermostat to 74 to 76 degrees for sleep comfort, set fan to auto, ensure filter clean to prevent coil freeze that causes water leak, and use programmable thermostat to raise temperature slightly. Leaving AC on all night is safe and does not shorten lifespan if maintained.
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