Every evening, Sleep Forecast publishes a single number for every major UK town: a Sleep Score from 0 to 10. Higher is better. A 9 means you'll sleep comfortably with the window open. A 2 means you're in for a rough night.
Here's how we calculate it.
Three measurements, one score
The Sleep Score is built from three overnight weather measurements, each weighted by how much impact sleep research says it has on your ability to fall and stay asleep.
Overnight minimum temperature (50% of the score). This is the main driver. Sleep researchers consistently find that the ideal bedroom temperature is between 16°C and 18°C. Above 20°C, most adults start to struggle. Above 22°C, it becomes genuinely difficult to fall asleep. We measure the lowest temperature forecast between 10pm and 7am.
Dew point (30% of the score). This is the one most weather tools miss. We use dew point instead of relative humidity because dew point is an absolute measure of how much moisture is in the air — it doesn't change with temperature the way relative humidity does. A dew point above 16°C feels uncomfortable. Above 18°C, it's oppressive — your sweat can't evaporate properly, so your body can't cool itself even with a fan.
Overnight cooling (20% of the score). How much the temperature drops between bedtime and 4am. Your body expects a dip overnight — your core temperature naturally falls as part of the circadian rhythm. When the air temperature doesn't drop, your body can't complete this cycle, and you wake up. Tropical nights where the temperature barely moves are the most disruptive kind, even when the absolute temperature isn't that extreme.
Why dew point, not humidity?
Most weather apps show relative humidity, which sounds useful but can be misleading. Relative humidity is relative to the current temperature — cold air with 80% RH can feel perfectly comfortable, while warm air with 60% RH can feel oppressive.
Dew point cuts through this. It tells you the actual amount of moisture in the air, regardless of temperature. Here's a rough guide:
| Dew point | How it feels |
|---|---|
| Below 10°C | Dry and fresh |
| 10–14°C | Comfortable |
| 14–16°C | Starting to feel muggy |
| 16–18°C | Uncomfortable — sticky |
| Above 18°C | Oppressive — sleep will be difficult |
When Sleep Forecast warns about "sticky air," we're talking about dew point, not humidity. It's the honest number.
The extreme heat ceiling
In July 2022, the UK recorded its highest-ever overnight temperatures. Parts of London and the South East didn't drop below 25°C all night. Our scoring includes an extreme heat ceiling for nights like these:
- Overnight low above 25°C: maximum score of 0, regardless of other conditions
- Above 24°C: maximum score of 1
- Above 23°C: maximum score of 2
- Above 22°C: maximum score of 3
This means a night where the temperature stays at 25°C will score 0 even if the dew point is low and the air cools significantly. At those temperatures, the heat alone is enough to prevent sleep for most people.
Where the data comes from
All weather data comes from Open-Meteo, an open-source weather API that aggregates data from national weather services including the Met Office, DWD, and NOAA. We use their hourly forecast for temperature, dew point, humidity, and wind speed.
Forecasts update every 30 minutes. The score you see at 6pm will be different from the one you see at 11pm as the forecast refines its overnight predictions.
What the score doesn't include
The Sleep Score measures outdoor conditions. Your actual bedroom will differ depending on insulation, which direction your windows face, whether you have a fan or air conditioning, and how many people and devices are in the room generating heat.
As a rule of thumb, bedrooms are typically 2–3°C warmer than the outdoor temperature, with a 2–4 hour lag. So when we say the overnight low is 19°C, your bedroom might not drop below 21°C until 2am.
That's why our advice is specific about when to open windows, not just whether to. Timing matters.