科学家研究发现，一天中喝咖啡的最佳时间大概是在上午9:30到11:30之间。这是由咖啡因和皮质醇相互作用的方式决定的。“皮质醇”帮助调节身体内部的生物钟，并且让人更加警觉。

皮质醇的水平在刚起床时是很高的，并且一直持续一个小时之久，在上午8点到9点达到最高。在皮质醇水平高的时候喝咖啡，会让人们对其中的咖啡因产生耐受性，意味着人们需要在早上再喝一杯咖啡才能达到相同的效果。

其他科学家们发现，在下午喝咖啡能帮助消化午餐吃得过多的食物。最近的调查表明，科学家是所有职业中喝咖啡最多的人，其次是市场营销和公关人士。

The best time to drink a cup of coffee to get your daily hit of caffeine is between 9.30am and 11.30am, according to neuroscientists.

It may seem that a cup of strong coffee first thing in the morning is the best way to get yourself going, but it in fact waiting until later may be better.

Scientists have worked out that the best time to get a daily hit of caffeine is on average between 9.30am and 11.30am.

This is due to the way caffeine interacts with a key hormone – called cortisol – which helps to regulate the body’s own internal clock and promotes alertness.

Cortisol levels are naturally high shortly after waking up and can remain high for up to an hour afterwards – with the average peak being between 8am and 9am.

Steven Miller, a neuroscientist at the Uniformed Services University of the Health Sciences in Bethesda, Maryland, argues it is better to then drink caffeine after this peak as it promotes cortisol production.

He said that drinking coffee while levels of this hormone are high can cause people to develop a tolerance of the caffeine it contains, meaning they often need an extra shot in their morning cup to get the same effect.

Writing as part of a light-hearted look at the evidence on his blog, he said: “If we are drinking caffeine at a time when your cortisol concentration in the blood is at its peak, you probably should not be drinking it.

“This is because cortisol production is strongly related to your level of alertness and cortisol peaks for your 24 hour rhythm between 8 and 9am on average

“One of the key principles of pharmacology is use a drug when it is needed. Otherwise, we can develop tolerance to a drug administered at the same dose.

“In other words, the same cup of morning coffee will become less effective and this is probably why I need a shot of espresso in mine now.”

Mr Miller, a neuroscience PhD student, wrote the advice after examining the evidence for how cortisol changes through the day.

Cortisol, which is produced in high levels in times of stress, helps to convert stores of energy into the sugar glucose so that it can be used by the body’s cells.

A burst of energy release in the morning leads to increased alertness but also increases feelings of hunger.

Scientists have also found that cortisol is also responsible for synchronising the internal daily rhythms that operate in the body.

Several studies have shown that cortisol levels increase rapidly shortly after waking and can stay elevated for up to an hour afterwards.

Other scientists have found that drinking coffee in the afternoon can help to offset lapses in attention that occur after a large lunch.

Recent surveys have also revealed that scientists are the heaviest coffee drinkers of all professions, closely followed by those who work in marketing and PR.

Editors and writers come in fourth place behind those working in education while doctors and health care workers feature further down the list.

Mr Miller said: “Although your cortisol levels peak between 8 and 9am, there are a few other times where blood levels peak again – between noon to 1pm, and 5.30 to 6.30pm.

“In the morning then, your coffee will probably be the most effective if you enjoy it between 9.30am and 11.30am, when your cortisol levels are dropping before the next spike.”

However, the exact time of when to drink that morning cup of coffee will vary from person to person depending on their cortisol cycle and the time when they normally get up in the morning.

Early risers may find their ideal time is earlier than those who get to enjoy a lie in.

（来源：中国日报网爱新闻iNews 编辑：丹妮）