Hygrometer
The standard answer, and the better one
30–50% relative humidity is the range recommended for occupied spaces, and bedrooms are not a special case. It is the same band that keeps people comfortable and mould suppressed.
But in summer that number is not the one that predicts a bad night, because a bedroom's relative humidity changes all night as the room cools. The figure that stays put is the dew point, and above about 65°F it reliably means a difficult night.
Why humidity affects sleep at all
Falling asleep depends on your core temperature dropping by around half a degree. That drop is not incidental to sleep onset — it is part of the mechanism.
The body achieves it largely by moving blood to the skin and letting heat escape, much of it through evaporation. Humid air slows evaporation, so the cooling takes longer or does not fully happen, and sleep onset is delayed and fragmented.
Which is why a humid 24°C night feels far worse than a dry one at the same temperature. The thermometer is identical; your ability to shed heat is not.
The overnight floor
The single most useful thing about the dew point here: air cannot cool much below it.
As the temperature falls to meet the dew point, condensing water releases latent heat almost as fast as the ground loses it, so cooling nearly stops. Tonight's dew point is therefore a good first guess at tonight's low.
| Dew point | The night will |
|---|---|
| Under 55°F | Cool down properly. Comfortable. |
| 55–60°F | Cool to around 60°F. Fine for most people. |
| 60–65°F | Stay close. Noticeably warm by early morning. |
| 65–70°F | Not fall below 65°F. Sleep quality drops for most people. |
| Over 70°F | Stay warm all night. No relief from the dark. |
This is why "it will cool down tonight" is often wrong on exactly the nights you most want it to be right — a muggy afternoon means a high dew point, and a high dew point means a floor under the low.
The window decision
The instinct on a warm night is to open a window. Whether that helps depends on what the outdoor air is carrying.
- Outdoor dew point lower than indoors — the incoming air is genuinely drier. Open it.
- Outdoor dew point higher — you are exchanging your air for air carrying more water. It will feel cooler for a few minutes as moving air evaporates sweat, then worse.
That second case is letting the heavy air in, and it is common on exactly the nights when opening the window feels most necessary. The outdoor dew point is what settles it, and this site shows it.
The dry end
Below about 30% causes its own problems: a dry throat on waking, a blocked nose, irritated airways, and worse snoring as membranes dry out. This is a winter issue, and it is the same arithmetic in reverse — cold outdoor air carries almost no water, and heating it adds none.
Not medical advice
This describes a mechanism. Persistent sleep problems have many causes and are worth taking to a doctor rather than attributing to the weather.
What this site can tell you
The outdoor dew point and how it will move overnight — which is what sets the floor under the low and what decides the window question. That is genuinely the more useful half.
For the bedroom itself the app reads a HomeKit humidity sensor if you have one, and otherwise estimates the likely range for a room of that kind from the outdoor conditions. That is enough to tell you whether the room is broadly dry or broadly damp; it is not a reading, and no phone has a humidity sensor of its own.