A leading group of sleep scientists is sounding a wake-up call: Hotter summer nights are stealing your sleep — and the problem is getting worse.

In a new analysis published in the journal SLEEP, the researchers found that heat is reshaping how billions of us rest.  On a night when the temperature stays at 81 degrees Fahrenheit or hotter, an additional 9,300 out of every 100,000 people get less than six hours of sleep.

(MORE: Heat index and the danger of high temperatures)

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Why nights are the real problem

Here’s the part that may surprise you: Summer nights are warming faster than summer days.  Across the United States, nighttime temperatures have warmed by just over 3 degrees on average since 1970, according to Climate Central. 

That’s a huge deal because nighttime is when our bodies are supposed to cool off and recover. If the air doesn’t cool off that reset never fully happens.

Weather.com senior meteorologist Jonathan Belles adds, “Part of the reason that sleepy time is warming faster than the afternoons is that humidity is also increasing. Muggy nights don't allow temperatures to drop as much, and the extra moisture doesn't allow the body to expel heat either … especially in sheets.”

To drift off and stay asleep, experts tell us, your core body temperature needs to drop by about 2 degrees, but when it stays elevated, sleep becomes shallow, broken and short.

And the damage doesn’t reset each morning. As homes soak up heat over the course of a heat wave, the loss compounds night after night — you start the next evening already behind, in a bedroom that never fully cooled down. 

(MORE: Heat waves can leave homes dangerously hot) 

People without air conditioning, older adults and those in the world’s hottest regions feel it first and worst.

How to sleep through a hot night

You can't change the forecast, but you can make your bedroom work against the heat instead of trapping it. A few things that help:

• Keep curtains closed during the day to block out the sun and keep heat from building up inside.
• Open windows on opposite sides of your home at night to create a cross-breeze that flushes warm air out.
• Swap in breathable cotton sheets that let your skin release heat instead of holding it in.
• Take a cool shower before bed to kick start that drop in body temperature.
• Stash your pillowcase in the freezer for a few minutes before you turn in.

(MORE: 7 cool finds to conquer the heat)

Researchers in the study say that across a warming planet, sleep loss is expected to not just increase, but accelerate, and they call it a public-health issue hiding in plain sight.