When you go back to work or class after lunch, why is it so hard to stay awake?

　　Although we get energy from food, we also use lots of energy to digest it. Your energy use increases 25-50% as you digest a meal. With all that action going on in the digestive system, the body sends extra blood to the gut to supply oxygen and other necessities.

　　How does the body know when to do this?

　　As food enters the stomach, the intestines secrete substances that dilate, or expand, the blood vessels that supply them with blood. As a result, more blood flows to the upper digestive tract, and there’s a bit less to go around to the rest of the body. That might make your arms and legs and brain feel a little sluggish!

　　At the same time, the small intestine releases a hormone that activates the part of the brain that controls sleep, making you drowsy. Finally, the natural components in foods can also contribute to the snooze effect. Certain foods, especially carbohydrates, increase production of serotonin in the brain. Serotonin works both to stop sugar craving, and to make you very relaxed. The larger the meal, the more pronounced that feeling can be. With all of these physiological reasons why we might feel sleepy after eating, a siesta after lunch sounds like a brilliant idea, and maybe after dinner and breakfast too!

　　午休的学问

　　当我们用完午餐回去工作或上课时，为什么很难保持清醒呢？

　　尽管我们从食物中获取能量，但也需要耗费许多能量来消化这些食物。消化一餐就会增加百分之二十五至五十的能量消耗。有了消化系统内的一系列活动，身体才能源源不断地将额外的血液输送到消化道以提供充足的氧气和其它必需物。

　　身体如何知道何时这样做？

　　当食物进入胃，肠道便分泌许多物质，它们使得为肠道供血的血管膨胀或扩张。结果便是更多的血液流向上消化道，而循环于身体其它部分的血液则相应减少，这时你可能感觉浑身懒洋洋的，连大脑也变得迟钝起来。

　　同时，小肠释放出一种荷尔蒙，它可以激活控制睡眠的大脑区域，让你昏昏欲睡。最后，食物中的天然成分也会让你打盹。某些食物，特别是碳水化合物，会增加大脑中血清素的生成。

　　血清素既可以抑制吃甜食的欲望，又可让你倍感放松。吃得越多，这种感觉就越明显。鉴于诸多生理原因导致我们餐后昏昏欲睡，也许午餐抑或早晚餐后小憩一会儿是个明智的选择。