39°C Heatwave in Korea: Smarter AC Use and Sleep Tips

39°C after "the start of autumn"

On Korea's traditional lunar calendar, early August marks ipchu — the nominal beginning of autumn. This year the calendar looks like a joke: daytime highs are still reaching 39°C (102°F), with the highest-tier heat warnings in effect across the Seoul metropolitan area, Gangwon, and parts of the Jeolla provinces. Nights aren't offering relief either, as temperatures stay above 25°C — what Koreans call a "tropical night." Apparent temperatures have been hovering around 35°C, and high-seas wind and wave advisories are up south of Jeju and in the outer eastern South Sea, where swells can reach up to 5 meters.

For most households the practical question narrows down to two things: how to run the air conditioner without a shocking electricity bill, and how to actually sleep. Everything below can be done tonight, without buying anything.

The cooling-cost question depends on your unit

The most confusing advice circulating online is that some people insist you should cycle the AC on and off, while others say never turn it off. Both are partly right, because the answer depends on the compressor type.

Fixed-speed units (common in older models, roughly pre-2011 in Korea): the compressor only runs at full power. Once the set temperature is reached it shuts off, then blasts again at full power when the room warms. With these, cooling the room down quickly and then switching off — repeating as needed — often works out cheaper.
Inverter units (most models sold today): the compressor slows down to a low, steady output once the target is reached. Frequent restarts waste energy here, so it's generally more efficient to leave it running steadily at a moderate setting.

If you don't know which you have, check the outdoor unit's label or search your model number for the word "inverter." One rule applies to both types: when you first switch on, set a lower temperature with the fan on high to pull the room temperature down fast, then raise the setting by a degree or two and hold it. Sitting at 18°C all evening is far more expensive than raising the fan speed at a moderate temperature.

Why a fan belongs next to your AC

Running a circulator or standing fan alongside the AC spreads cooled air through the room, letting you set the thermostat 1–2°C higher for the same perceived comfort. Counterintuitively, aiming the circulator at the opposite wall — rather than at yourself — tends to work better, because it creates a circulation loop. Also check the outdoor unit: if it's boxed in by stored items or crammed into a tight space, it can't dump heat, and efficiency drops.

Getting through a tropical night

Nighttime heat is a different problem from daytime heat. Falling asleep requires your core body temperature to drop slightly, and a hot bedroom interferes with that.

Pre-cool the bedroom an hour or two before bed, then use a sleep timer (1–3 hours) so you don't over-cool at 4 a.m.
Take a lukewarm shower rather than a cold one. Cold water feels good instantly but constricts blood vessels, which can slow heat release.
Swap in breathable bedding; if you use a cooling pad, laying a thin sheet over it reduces the clammy feeling.
Drink water in small amounts before you feel thirsty. Caffeine and alcohol work against you overnight because of their diuretic effect.

Older adults and people with chronic conditions are especially vulnerable at night, since the body's temperature regulation is less responsive. If you have an elderly relative living alone, a quick check-in each evening and morning meaningfully lowers the risk. Headache, dizziness, nausea, or sweating that suddenly stops are signals to move to a cool place and contact medical help immediately — heat illness is not something to tough out on your own, and a professional should be consulted.

Reshuffle your outdoor schedule

On days with an active heat warning, the single most effective measure is simply reducing exposure. Shift outdoor work or exercise away from 2 p.m. to 5 p.m., the hottest window, to early morning or after sunset. If outdoor work is unavoidable, break in the shade every 20–30 minutes and take in a little salt along with water. In Korea, local governments operate designated cooling shelters (muduwi swimteo), and banks or community service centers also work as air-conditioned refuges — worth locating in advance.

If you're planning a weekend by the water, check marine forecasts too. With advisories currently in effect south of Jeju and in the outer eastern South Sea, waves up to 5 meters make small-boat trips and rock fishing genuinely dangerous.

One small laundry trick for summer

Summer means more sweat and more spills. Kimchi juice is a notoriously stubborn stain because it combines pigment and oil, so ordinary washing rarely lifts it. A long-standing Korean household trick: pack grated onion onto both sides of the stained area, leave it for about a day, then launder as usual. Test it on a hidden seam first, since some fabrics can discolor — and for delicate materials like silk or wool, a professional cleaner is the safer call.

The takeaway

This summer's heat has made seasonal intuition useless as a planning tool. Protecting both your budget and your health isn't about refusing to run the AC; it's about knowing which type you own and holding a moderate temperature — roughly 26–28°C — while using fan speed and air circulation to manage how cool it feels. At night, let the timer do the work so your body gets a real chance to recover.