Not because we’re more exhausted – it’s because the air is cooler. According to the journal Frontiers in Evolutionary Neuroscience, we yawn to keep our brain at the top of its game. Because when we inhale deeply, we’re replacing the hot, stagnant air in our lungs with the cool, fresh air around us. And the brain operates best when it’s cool – so yawning draws in cool air, and makes us more alert and quicker thinkers. So we yawn more in winter than summer because the air is cooler.