5 Reasons Your Aircon Is Not Cold at Night but Fine During the Day

During the day, most people run the aircon in bursts — a few hours at a time with breaks in between. At night, the unit runs continuously for the full sleep duration. That sustained load exposes weaknesses that short daytime runs hide. A compressor that cools fine for a couple of hours may struggle after six or seven.

Night also changes the moisture profile of the room. Closed doors, body heat, and reduced ventilation raise the humidity load. The aircon has to remove more moisture from the air before the room feels cool, which means the coil works harder even if the outdoor temperature is lower.

A sealed bedroom with one or two people sleeping produces a steady stream of moisture through breathing and perspiration. During the day, opening doors and moving between rooms gives moisture a chance to disperse. At night, the room stays closed, and humidity accumulates faster than the aircon can extract it.

High humidity makes the air feel warmer even when the thermostat reads the right number. The aircon may be reaching its set temperature, but the room still feels sticky and warm. This is not a fault — the unit is doing its job on temperature but struggling with the moisture load. Running the unit in dry mode for the first part of the night can help the coil prioritise dehumidification.

The outdoor unit relies on air flowing across the condenser coil to reject heat. During the day, natural wind and thermal currents help move air around the unit. At night, wind speeds drop, especially in sheltered HDB corridors and condo ledges. The condenser has less airflow to work with, so it rejects heat less efficiently.

The effect is more noticeable when the outdoor unit is installed in a tight space or behind louvres that restrict airflow. During the day, even a slight breeze compensates. At night, the unit relies entirely on its own fan, and if the space is enclosed, the hot air recirculates around the condenser instead of dissipating.

The thermistor is the sensor that tells the PCB board what the room temperature is. Some thermistors lose accuracy after hours of continuous operation — the reading drifts slightly, and the PCB thinks the room is cooler than it actually is. The unit reduces its output or cycles off, and the room warms up.

This drift is small — often just a degree or two — but enough to make the difference between comfortable and too warm at three in the morning. A technician can test the thermistor by comparing its reading to an independent thermometer after the unit has run for several hours. If the readings diverge, the sensor needs replacing.

Many units have a sleep mode that gradually raises the set temperature overnight. The logic is that your body needs less cooling as it enters deep sleep. But if the ramp is too aggressive, the room temperature climbs noticeably by the second half of the night. Some units raise the setpoint by a degree every hour — after several hours, the room is significantly warmer.

A timer set to turn the unit off at a fixed hour can cause the same pattern. The room stays cool for the first few hours, then warms after the unit stops. If the cooling loss always happens around the same time, check the timer and sleep mode settings before suspecting a hardware fault.

A compressor that is starting to wear can handle short runs fine but weakens under continuous operation. After several hours, internal friction builds heat, oil viscosity changes, and the compressor's pumping efficiency drops. The cooling output falls gradually — not a sudden stop, but a slow fade.

This pattern is the hardest to confirm because it looks like so many other things. The key indicator is that the room reaches temperature within the first hour but slowly warms after that, even though the unit keeps running. A technician measuring discharge pressure after sustained operation can spot the decline. If the pressure drops while the unit is running and the condenser is clean, the compressor is not maintaining its output.

Start with the simple checks. Review your timer and sleep mode settings. Clean the filter if it has not been done recently. Make sure the outdoor unit has clearance and is not boxed in by stored items or drying laundry.

If those checks do not change anything, the issue is likely humidity load, thermistor drift, or compressor wear — all of which need a technician to confirm. Describe the pattern clearly: what time the cooling drops, whether the unit is still running when it happens, and whether the problem is consistent every night. That detail helps the technician focus the diagnosis.