Around 2 pm on weekdays, most people will find they suddenly lack of motivation at work. Apart from this, the overwhelming sense of embarrassment from the brain makes you feel distracted and just want to have a cup of coffee to save yourself. An article, published in the Scientific American Chinese Edition,introduced a new research, which explained this phenomenon from the angle of the brain reward mechanism: the brain focused on looking for a reward, and the reward can bring the power, so that people can continue to work after taking anap after lunch. First of all, what does "reward" mean here?. That is to say, we get up early, go to work, finish the daily tasks, and so on. The reasons for doing all kinds of things are for reward -- everything that makes us try to achieve, to strive for, to learn, and to change. The brain is born to spend almost all of its time focusing on rewards. The study shows that thebrain's quest for reward rises and falls within a day.

The researchers used fMRI (functional magnetic resonance imaging)（功能性核磁共振影像技术）to scan the brain activity of a group of volunteers at 10 a.m, 2 p.m., 7 p.m. , and found a area named left putamen(左侧豆状) in the brain, which moved frequently both in the morning and in the evening , but remained peaceful in the afternoon. Is the most quiet. The left putamen region plays an important role in reward processing (奖赏处理)and reward anticipation(奖励预期) in the brain.

The researchers speculate that the reason for this change is that there exists prediction error in reward anticipation in the brain. As for the expected reward value(奖励预期值), the brain will be in the low tide period in the morning andevening, as a result of which, the brain demands more reward to go on working.In other words, when you think of what theprobability is low, you are willing to spend extra effort getting it. Theexpected value will peak at noon, and then the momentum of the effort is lost.Forecasting errors are important because our expectations do not match withthose actual rewards, so we want to learn or accomplish more during this period.

Although the researchers' explanations are only based on speculation, it is suggested that we shoul better not to finish the big projects at noon.

为什么我们一到下午就想偷懒

工作日下午两点左右，大多人会发现自己突然没动力干活了，那种来自大脑压倒性的困顿感让你忍不住地走神，只想喝一杯咖啡解救自己。《科学美国人》中文版发表的一篇文章中，介绍了一项新研究，这个研究从大脑的奖赏机制角度给予了解释：大脑专注于寻找奖赏，奖赏能带来动力，让人们在午休后能继续工作。首先要知道，这里的“奖赏”指的是什么。可以这么说，我们早起，去上班，完成每日任务等等，做各种事的原因都是为了奖赏——那些使得我们努力实现、争取、学会、成就以及改变的一切。大脑生来就会把几乎所有时间都放在关注各种奖赏上。这项研究显示，大脑对奖赏的追求程度在一天之内会有起伏。

研究人员利用fMRI（功能性核磁共振影像技术）扫描了一小组志愿者在早上10点、下午2点、晚上7点的大脑活动，发现大脑里一个名为“左侧豆状核（left putamen）”的区域，在上午和晚上的活动最频繁，在午后则最为安静。左侧豆状核区域在大脑的奖赏处理和奖赏预期中发挥重要作用。

研究者推测发生这种变化的原因是，大脑对什么时候能预期奖赏存在“预测误差（prediction error）”。上午和晚上，是大脑对奖励预期值的低潮期，所以需求奖赏的动力会更高，也就说当你认为得到什么的概率比较低的时候，你愿意花额外的精力来得到它。预期值会在中午达到峰值，于是这时候就失去了努力的动力。预测误差有着重要意义——因为我们的期待值和实际会获得奖赏并不一致，所以我们才想在这段时间内学到或完成更多的事情。

虽然研究者的这些解释只是基于推测，但最好还是别选择在中午去完成那些大项目了。