The short answer
Sleep mode usually shifts your setting by a degree or two over the first hours of the night, warmer when cooling and cooler when heating, and turns the fan down. Quiet mode slows the fan to cut noise, at the cost of some output.
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Bedrooms are where noise, light and draughts matter most, so most systems have settings meant for the night. They go by names such as sleep, night, silent and quiet, and what each one does varies between models, so check your manual for the details.
The night-time settings compared
| Setting | What it usually does | The trade-off |
|---|---|---|
| Sleep | Shifts the setting gradually, lowers the fan | Room drifts from your daytime setting |
| Quiet (indoor) | Runs the fan at its lowest speed | Less cooling or heating reaches the room |
| Outdoor quiet | Limits the outdoor unit's speed | Less output on the hottest or coldest nights |
| Display off | Dims the lights on the unit | Harder to see what it's doing |
| Off timer | Switches off after a set time | The room warms or cools again |
Not every model has all of these, and some combine them. On some systems, choosing sleep also dims the display or ends after a set number of hours.
How sleep mode works
Sleep mode is designed to stop a room becoming colder than you need overnight when cooling, or warmer than you need when heating. It does this by moving the setting gradually. When cooling, it typically raises the setting by a degree or two over the first hour or two. When heating, it lowers it by a similar amount.
Most sleep modes also drop the fan to a low speed. Some run until you change them in the morning, while others finish after a set number of hours or switch the unit off. Check which yours does before relying on it through a hot night.
Quiet mode, indoors and out
Indoor quiet mode runs the fan at its lowest speed. Going by manufacturers' sound pressure figures, quiet indoor units are often around 19–26 dB(A) on that setting. For what those numbers mean, see decibels explained.
Moving less air means moving less heat. On a warm night a unit left on quiet may not hold the room at your setting, especially in a bedroom that has soaked up sun all day. It helps to cool the bedroom on a normal fan speed in the evening, with the door shut and the blinds closed, then switch to quiet or sleep when you go to bed.
Some systems also have a quiet setting for the outdoor unit, which limits how fast its compressor and fan run. On some models it can be set to come on automatically at night. It can help where the outdoor unit is close to a bedroom window, yours or a neighbour's, but it reduces the cooling or heating available too.
Sounds you may still hear
Even on the quietest settings, a working system makes some sounds that are perfectly normal:
- a gurgle or trickle as refrigerant moves, often when the unit starts or stops
- light clicks or creaks as plastic parts expand and contract with temperature changes
- the flap moving when the unit starts, stops or changes mode
- in heating, a hiss or whoosh when defrosting starts and ends, while the indoor fan pauses
Rattling, grinding, squealing or loud buzzing aren't normal. See noises: what's normal and what isn't.
Setting up for a comfortable night
- Point the airflow away from the bed so moving air doesn't blow across you. Level or slightly upwards usually works when cooling, as directing the airflow explains.
- Cool the room before bed, then switch to sleep or quiet mode.
- Use an off timer if you'd rather the unit stopped after you fall asleep, bearing in mind the room will warm up again on a hot night.
- Keep the windows shut while it runs, and the bedroom door closed if you can.
- Turn the display lights off if they bother you. The remote often has a button for this.
Related guides
- Sleeping in hot weatherBedrooms stay warm at night because the building gives back the heat it soaked up during the day. Keeping sun out by day, ventilating once it's cooler outside, and quiet cooling or a fan all make a warm night more comfortable.
- How loud is air conditioning?Quiet indoor units are often around 19–26 dB(A) on their lowest fan speed and louder at higher speeds, while outdoor units are typically 45–50 dB(A) at 1 metre. How loud it seems also depends on where the units are and how hard they're working.
- Timers and schedulesMost systems have on and off timers that count down a set number of hours, and many controllers and apps add clock times and weekly schedules. Used well, they have a room comfortable when you need it and the system off when you don't.