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In order to take a closer look, researchers monitored 36 participants as they watched other people yawn. Turns out it comes down to each person’s excitability.
Most people yawn because they’re tired, but it can also happen unexpectedly and without any triggers. While yawning is typically harmless and only lasts about five to 10 seconds, when it occurs ...
Merely thinking about or seeing someone yawning can make you yawn. But why?
The yawn reflex has long been observed to be contagious. In 1508, Erasmus wrote, "One man's yawning makes another yawn", [39] and the French proverbialized the idea to "Un bon bâilleur en fait bâiller sept" ('One good gaper makes seven others gape'). [40] Often, if one person yawns, this may cause another person to "empathetically" yawn. [23]
Each cycle is approximately 90 minutes long, containing a 20-30 minute bout of REM sleep. [7] NREM sleep consists of sleep stages 1–4, and is where movement can be observed. A person can still move their body when they are in NREM sleep. If someone sleeping turns, tosses, or rolls over, this indicates that they are in NREM sleep.
In a study where subjects were restricted to 4 hours of sleep per night for 2 nights, leptin levels decreased by 18% and ghrelin levels increased by 28%. In addition, there was an increase in hunger rating by 23%, with leptin levels being a significant predictor of hunger levels.
But there's a window: People yawn most when it's around 68 degrees. When it's really hot outside, people are less likely to yawn because it would have very little impact on the brain's temperature.
Why We Sleep: The New Science of Sleep and Dreams (or simply known as Why We Sleep) is a 2017 popular science book about sleep written by Matthew Walker, an English scientist and the director of the Center for Human Sleep Science at the University of California, Berkeley, who specializes in neuroscience and psychology.