For many of us, it is the rocket fuel that gets us going first thing in the morning.
But one expert has argued that to get the maximum effect of caffeine, it's better to hold on a few hours - because coffee is most effective if consumed between 9.30am and 11.30am.
Steven Miller, a neuroscientist from the Uniformed Services University of the Health Sciences in Maryland, says it is best to drink coffee when the body’s levels of the hormone cortisol are low because caffeine interacts with the hormone.
hormone：荷尔蒙 cortisol：皮质醇
He explained that people who drink coffee when their cortisol levels are high –namely, between 8am and 9am –develop a tolerance to caffeine, meaning it becomes less effective.
Cortisol controls the body clock and causes people to feel wide awake, The Telegraph reports. The body’s levels of the hormone are usually high just after a person wakes up but start to fall about an hour afterwards.
Mr Miller argues that it is during this drop in cortisol levels that people get the most benefit from the caffeine in coffee –it gives them a boost as it encourages cortisol production when their cortisol levels start to drop and they do not become resistant to its benefits.
He explained that cortisol levels also peak at lunchtime and between 5.30pm and 6.30pm, meaning that these would also not be good times to drink coffee.
Writing 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.
alertness：警戒，机敏
He added that people who drink a coffee first thing in the morning, when their cortisol levels are naturally high, will soon find themselves needing to make their coffee stronger and stronger to get the desired effect.
However, he did accept that cortisol levels at different times of day vary from person to person meaning that the best time of day to drink a coffee also differs between people.
For example, people who get up very early may find their cortisol levels drop earlier than those of people who get up late.
He says it helps turn energy into sugar to be utilised by cells in the body and that it also plays a role in controlling the body clock.