Air Conditioner: "Off Timer" or "Leave On" - Which is Correct? Experiment Verifies How to Best Use Air Conditioners on Hot Summer Nights, With "Off Timer" Causing Room Temperatures to Exceed 30°C in the Early Morning. A Difference of 15.2 Yen Per Night in Electricity Costs Improves Sleep Environment.

Panasonic Corporation conducted an experiment on air conditioner usage, room temperature, and power consumption.

According to Japan Meteorological Agency observation data, the number of tropical nights (minimum daily temperature 25°C or higher) from July to September 2025 is projected to be 54 days in Tokyo, 69 days in Nagoya, 75 days in Osaka, and 77 days in Fukuoka. This means over half the nights in Tokyo and around 80% of nights in Nagoya, Osaka, and Fukuoka will be tropical nights, with hot nights continuing for extended periods.

The Japan Meteorological Agency's three-month weather forecast (July-September), released on June 23, indicates that this summer, warm air masses are likely to prevail, leading to higher-than-average temperatures in eastern and western Japan, as well as Okinawa and Amami, and average or higher temperatures in northern Japan. This is expected to result in many uncomfortable nights.

Furthermore, since June 1, "sleep disorders" have been added to the list of medical specialties that healthcare institutions can advertise, highlighting the growing importance of sleep.

As the heat of summer nights intensifies and the need for household savings increases due to rising prices, Panasonic conducted this experiment to clarify "how much the sleep environment and power consumption change depending on how the air conditioner is used," which is crucial for everyone to consider optimal sleep measures.

【Summary of This Release】

"Off Timer" vs. "Leave On": A Comparative Experiment! "Off Timer" Leads to Temperatures Exceeding 30°C in the Early Morning, with a 15.2 Yen Per Night Electricity Cost Difference Compared to Leaving It On.

Professor Kazue Mizuno, an Expert in Bedroom Environment and Sleep, Explains! The Mechanism of "Mid-Sleep Awakening" and the Recommendation to "Leave the Air Conditioner On All Night."

How to Create a Comfortable Sleep Environment on Hot Summer Nights? Air Conditioner Experts & Sleep Improvement Instructors Share Tips on Air Conditioner Usage.

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"Off Timer" vs. "Leave On": A Comparative Experiment!

"Off Timer" Leads to Temperatures Exceeding 30°C in the Early Morning, with a 15.2 Yen Per Night Electricity Cost Difference Compared to Leaving It On.

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This experiment was conducted in our company's environmental test room (6 tatami mats), replicating daytime and nighttime outdoor temperatures and solar radiation. 140 sensors were suspended from the ceiling to measure the temperature of the "space" that people actually come into contact with. Additionally, a heat source equivalent to one person was placed in the test room to account for the impact of body heat on room temperature. Room temperature and air conditioner power consumption were measured every 10 seconds, and electricity costs were calculated from the power consumption.

1 Room Temperature Rise with a 3-Hour Off Timer

Reached 31°C after 8 hours.

In the nighttime simulation, the room temperature, initially at 30°C, decreased with the start of the air conditioner and stabilized after 30 minutes. When the air conditioner was left on, the temperature remained stable at 26-27°C for the entire 8-hour measurement period, indicating that the indoor environment was kept comfortable until early morning.

On the other hand, with the 3-hour off timer, the room temperature rose to 28.6°C just 30 minutes after the air conditioner turned off and continued to climb, reaching 31.2°C by the end of the 8-hour measurement. This revealed that even if people could fall asleep comfortably, the environment during most of their sleep time would be one that many would find uncomfortable due to the heat.

2 Electricity Cost Comparison: 3-Hour Off Timer vs. Leaving On

A Difference of 15.2 Yen for 8 Hours of Nighttime Use.

What is the difference in electricity costs between leaving the air conditioner on all night and using a 3-hour off timer?

Based on the power consumption measured in this experiment, the electricity cost for 8 hours was 26.7 yen (862.5Wh) when left on, and 11.5 yen (372.1Wh) with the 3-hour off timer, resulting in a difference of 15.2 yen.

3 Electricity Cost Comparison: Daytime vs. Nighttime Use

A Difference of 26 Yen for 8 Hours, as Nighttime Electricity Costs are Lower Due to Lower Temperatures.

Air conditioners consume different amounts of power during the day and night. This experiment also measured the power consumption and calculated the electricity cost for leaving the air conditioner on for 8 hours during the day. The result was 52.7 yen (1.7kWh), a difference of 26 yen compared to nighttime use (26.7 yen).

The main reasons for this difference are the outdoor temperature and the presence or absence of solar radiation. The electricity cost of an air conditioner is affected by the difference between the outdoor temperature and the set temperature. Raising the set temperature by 1°C in summer can save about 10% on electricity, but since the outdoor temperature at night is more than 5°C lower than during the day and there is no solar radiation, the cooling load at night is smaller, allowing the room temperature to be lowered with less power consumption. In this experiment, nighttime electricity costs were approximately half of daytime costs.

A video summarizing the results of this experiment has been released.

Experiment Overview

Measurement Items

Room temperature, power consumption

Measurement Time

8 hours

Measurement Method

・Evaluated under our company's unique conditions in our environmental test room (6 tatami mats).

Daytime: <Outdoor Conditions> Temperature 35°C / Humidity 40% <Solar Radiation> Present <Air Conditioner Setting> Cooling 28°C / Auto fan speed and direction.

Nighttime: <Outdoor Conditions> Temperature 30°C / Humidity 90% <Solar Radiation> Absent <Air Conditioner Setting> Cooling 28°C / Auto fan speed and direction.

・A heat source equivalent to one person was placed in the test room, assuming one person was present.

Model Used

Eolia Inverter Cooling/Heating/Dehumidifying Air Conditioner CS-J226D

*When citing the experimental results, please attribute them to "Research by Panasonic Eolia."

*The electricity costs described above are calculated based on a standard electricity rate of 31 yen per kWh (as determined by the Japan Electrical Manufacturers Association's standard electricity rate for household products). Actual power consumption and electricity costs may vary depending on the air conditioner's usage environment.

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Professor Kazue Mizuno, an Expert in Bedroom Environment and Sleep, Explains!

"Mid-Sleep Awakening" and the Recommendation to "Leave the Air Conditioner On All Night."

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Panasonic interviewed Professor Kazue Mizuno of Wayo Women's University, an expert in bedroom environments and sleep, about the mechanisms of mid-sleep awakening, its impact on daily life, and how to create a comfortable sleep environment.

*This comment was edited and published after being requested by Panasonic from Professor Mizuno. This comment reflects general knowledge and does not indicate the results of this experiment or the effectiveness of specific products.

You Might Think You're Sleeping, But You Might Not Be. Various Health Risks Associated with Mid-Sleep Awakening.

Awakening is determined by brain waves. If the brain waves indicate awakening, it is considered awakening, even if you are not consciously aware of being awake. If mid-sleep awakenings are too frequent and sleep efficiency decreases, the risk of lifestyle diseases such as depression, constipation, obesity, diabetes, and hypertension increases. It also lowers immunity, increasing the risk of injuries and accidents. In children, it can lead to decreased academic performance and reduced emotional regulation. In summer, the body's ability to regulate temperature during the day decreases, which may increase the risk of heatstroke (1) (not directly related to the experimental results).

Mid-Sleep Awakening Occurs When the Body Prioritizes Temperature Regulation Over Sleep. A Room Temperature of 28°C is the Borderline for Comfortable Sleep.

An increase in core body temperature during sleep is thought to lead to mid-sleep awakening. Since the body's temperature regulation function decreases during sleep compared to when awake (2), mid-sleep awakening can be considered a state where the body prioritizes temperature regulation over sleep (i.e., the temperature regulation function reaches its limit during sleep). Previous research suggests that the upper limit for comfortable sleep temperature is 28°C (3). Although there are individual differences, consider using air conditioning if the temperature exceeds 28°C. Humidity decreases with air conditioner operation, so it is generally advisable to prioritize temperature control.

Lowering Room Temperature is Key to Comfortable Sleep. If You Feel Hot or Uncomfortable, Leave the Air Conditioner On All Night.

Our research has also shown that using the air conditioner only in the first half of the night results in more mid-sleep awakenings compared to using it all night (4). Fewer awakenings and fewer times waking up indicate that sleep was maintained well. Even if you use an off timer to save electricity, you might wake up due to the heat and turn the air conditioner back on, leading to frequent on-off cycles and ultimately poor sleep. If you anticipate it getting too cold, we recommend having extra bedding or blankets available for your feet. In hot environments, it is difficult to sleep comfortably with only bedding and clothing. If you feel hot or uncomfortable before going to bed, do not hesitate to use the air conditioner. This is especially recommended on tropical nights. Lowering the room temperature is a crucial point for comfortable sleep.

1) Mizuno, K., & Horie, Y. (2021). Relationship between Sleep, Health, and Heatstroke: Comfortable Sleep Methods for Surviving Summer. Local Government Officials' Safety and Health Forum, 31(3), 10-13.

2) Jennings, J. R., REYNOLDS III, C. F., Bryant, D. S., Berman, S. R., Buysse, D. J., Dahl, R. E., & Monk, T. H. (1993). Peripheral thermal responsivity to facial cooling during sleep. Psychophysiology, 30(4), 374-382.

3) Yanase, S. (1999). Thermal Environment. In T. Torii (Ed.), Sleep Environment Science (pp. 152-157). Asakura Shoten.

4) Okamoto-Mizuno, K., Tsuzuki, K., & Mizuno, K. (2005). Effects of humid heat exposure in later sleep segments on sleep stages and body temperature in humans. International journal of biometeorology, 49(4), 232-237.

Kazue Mizuno

Professor, Department of Human Environment, Faculty of Human Development, Wayo Women's University. Previously held positions at Dokkyo University School of Medicine, Department of Physiology, as a NEDO Fellow at the National Institute of Advanced Industrial Science and Technology, and as a Specially Appointed Researcher at Tohoku Fukushi University. Engaged in research on the effects of bedroom heat and cold, bedding, and sleepwear on sleep, pursuing optimal thermal environments for good sleep. Member of the Council of the Physiological Anthropology Society of Japan and Director of the Society for Sleep Environment Research.

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How to Create a Comfortable Sleep Environment on Hot Summer Nights?

Air Conditioner Experts & Sleep Improvement Instructors Share Tips on Air Conditioner Usage.

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Fuko Fukuda, Panasonic Air Master and Sleep Improvement Instructor, explains how to use air conditioners to improve sleep quality during the summer.

*Sleep environments vary individually and depend on air conditioner installation and usage conditions.

1 Turn on the Air Conditioner 30 Minutes Before Bedtime with the Airflow Directed Upwards.

Generally, a room temperature of 26-28°C is considered comfortable for sleep. To maintain a comfortable bedroom environment, use the cooling mode set between 26-28°C or the dehumidifying mode. The timing of air conditioner operation is as important as the temperature. In this experiment, the room temperature dropped below 28°C within 30 minutes of the air conditioner starting. Turning on the air conditioner 30 minutes before entering the bedroom and directing the airflow upwards can efficiently create a good sleep environment.

2 Don't Forget to Clean Your Bedroom Air Conditioner Filter.

Bedroom filters are prone to accumulating dust, especially from fibers shed from bedding and clothing. A clogged filter reduces airflow, decreasing cooling/heating efficiency, leading to wasted electricity, and increasing the risk of unit malfunction. To maintain a comfortable sleep environment, it is recommended to clean the filter approximately once every two weeks. Vacuum the dust and, if heavily soiled, wash it with water and dry it thoroughly.

3 Control Humidity, Not Just Temperature, for Greater Comfort! Maintain Humidity Below 60%.

Excessive humidity can cause awakenings during the night. Pay attention to humidity as well as room temperature. It is important to keep bedroom humidity below 60%. If the humidity is high, it is recommended to lower the air conditioner's temperature setting or use the dehumidifying mode.

4 Use in Conjunction with a Fan.

If the room temperature is too high and you have trouble falling asleep, using a fan in conjunction with the air conditioner is recommended. In this case, directing the airflow towards the ankles, where large blood vessels are close to the surface, can help lower core body temperature and facilitate sleep.

*Do not direct the airflow onto your body for extended periods, as it may be harmful to your health.

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Features Recommended for Bedrooms

Panasonic Eolia (EX Series) with Compact Depth

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"Filter Cleaning Robot" Automatically Cleans the Filter* on Eolia.

The "Filter Cleaning Robot" feature on Panasonic's Eolia air conditioners (EX Series) automatically starts cleaning after accumulating over 24 hours of operation. The filter moves on its own, and brushes densely arranged on it thoroughly remove dust from the filter. It then scrapes off dust from the brushes and automatically discharges it outdoors (in the case of automatic discharge type), eliminating the need to empty a dust collection box.

*For use in environments with heavy dust or oil stains, we recommend removing and washing the filter with water.

Comfortable Dehumidification Mode.

The air conditioner checks both room temperature and humidity to provide comfortable control. It achieves comfortable dehumidification that is less cooling-intensive than the cooling-dehumidifying mode, effectively removing moisture while maintaining a dry and comfortable environment.

*Room temperature may decrease depending on the usage environment (this is not a reheating dehumidification system).

Click here for Eolia 2025 models (EX Series):

https://panasonic.jp/aircon/EXseries.html

Click here for Eolia's cleaning performance, including the "Filter Cleaning Robot":

https://panasonic.jp/aircon/feature/clean.html

Click here for Eolia's dehumidification functions, including "Comfortable Dehumidification Mode":

https://panasonic.jp/aircon/feature/dehumidification.html

【Fuko Fukuda, Panasonic Air Master】

Panasonic HVAC & CC Co., Ltd.

Has installed four different models of air conditioners at home, distinguishing between their functions and airflow. She remotely controls air conditioners throughout her home using her smartphone and has even acquired and disassembled air conditioners with mold to investigate them, going beyond her assigned duties as a bit of an air conditioner enthusiast.

Panasonic Air Conditioner "Eolia" is Certified as the Official Air Conditioner for the "Zero Heatstroke" Project. The "Zero Heatstroke" project aims to reduce the number of people suffering from heatstroke and eliminate fatalities. https://panasonic.jp/aircon/contents/zero_heatstroke.html