Ever wondered why you sometimes yawn even if you're not feeling sleepy? As far as science goes, people yawn because they have diminished oxygen in their body.

However, certified personal trainer Sarah Klein said there's actually other reasons for yawning. She wrote for Prevention that people yawn whenever they are with somebody they like. Klein said yawns are contagious, and it's also a way for people to display empathy for one another. "In a 2011 study, researchers found that yawns were most contagious between family members, followed by between friends, and least contagious between strangers," she said.

Another reason why people yawn is that their brains need cooling. People yawn more during cooler months and less when the temperature is sizzling, said Klein. "The cooling of the brain would in turn give us the extra energy we need in moments when we let out a big yawn," she explained.

Surprisingly, she said people with bigger brains yawn more. A recent report from the journal Biology Letters stated that mammals that let out big, long yawns have heavier brains and higher number of brain cells.

"Assuming that yawns do indeed cool the brain in order to energise it, bigger brains with more neurons would require more oxygen to wake things up, therefore resulting in bigger yawns, the thinking goes," she explained.

The last reason for yawning is that people might be having a heart attack or stroke. Before people get alarmed, Klein clarified that this is only for those who are yawning excessively. She added that these conditions are linked to problems regulating brain temperature, so the body yawns to help the body out.

"Heart attacks can stimulate the vagus nerve, which runs from the brain to the abdomen, leading to a reaction that could trigger excessive yawning," she said. "People with epilepsy and multiple sclerosis also often report frequent to excessive yawning."