科學家研究發現，一天中喝咖啡的最佳時間大概是在上午9:30到11:30之間。這是由咖啡因和皮質醇相互作用的方式決定的。「皮質醇」幫助調節身體內部的生物鐘，並且讓人更加警覺。
皮質醇的水平在剛起床時是很高的，並且一直持續一個小時之久，在上午8點到9點達到最高。在皮質醇水平高的時候喝咖啡，會讓人們對其中的咖啡因產生耐受性，意味著人們需要在早上再喝一杯咖啡才能達到相同的效果。
其他科學家們發現，在下午喝咖啡能幫助消化午餐吃得過多的食物。最近的調查表明，科學家是所有職業中喝咖啡最多的人，其次是市場營銷和公關人士。
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.
（來源：中國日報網愛新聞iNews 編輯：丹妮）