The downside of hot summer days are hot summer nights. When the temperature doesn't drop below 20 degrees Celsius (68 degrees Fahrenheit) at night—as is currently the case in many parts of Europe and North America—we become restless. We toss and turn in bed for hours, find it difficult to fall asleep, and feel groggy the next day. Sound familiar?
This has mainly to do with how closely sleep and the body’s temperature regulation are linked. Our internal temperature, which is normally around 37 degrees Celsius, naturally drops a little at night to make us fall asleep. About 1 degree of heat is redistributed from the core of the body to the hands and feet, which have large surface areas and specialized blood vessels to allow this heat to dissipate. The hormone melatonin plays an important role in this: When it’s dark, melatonin is secreted from the pineal gland in the brain and serves as a timer for our internal clock. It widens the blood vessels in the hands and feet to allow the body to rid itself of heat faster and help us nod off.
That is, if the ambient temperature doesn’t mess things up. The ideal bedroom temperature for adults is somewhere between 15 and 19 degrees Celsius (59 and 66 degrees Fahrenheit), depending on the person, and the body has to work harder to regulate its own temperature when this isn’t achieved. And if the room temperature doesn’t fall sufficiently after a hot day, then our ability to regulate our body temperature is impaired. Not only do we then have trouble falling asleep, but the hot air can interrupt our sleep stages, too.
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