Hi Hazel, so I've been told that you're not supposed to have coffee past 12pm because of

it's half-life and how it'll affect your sleep but I'm just wondering for somebody who does an afternoon

workout is it okay to drink coffee before that if you're going to be burning off that energy

will it affect the half-life thank you This is a great question and actually I didn't

know the answer myself so I'm really glad that you asked. There are a lot of papers on the effects

of caffeine on exercise but not many the other way around. Caffeine has some pretty powerful effects

when it comes to mental and physical performance,

but the timing and the dose does matter. So first of all, before we jump into the question,

how does caffeine work? So it's a psychoactive stimulant and it works through a variety of

mechanisms, but mainly through its effects on adenosine.

So adenosine accumulates across the day as we're awake, the more we have, the sleepier we feel.

Then caffeine comes in and acts by preventing adenosine binding to its receptors.

So essentially it's elbowing caffeine out of the way and blocking it from

doing its job. So this increases alertness, which of course can be a positive thing depending on

the time of the day, but it can also have a knock on effect to your sleep later that night.

So the half time, which is the time taken to eliminate half the caffeine from

your body, varies from person to person, but on average is about five hours, although research

shows that it can be anywhere from two to ten hours, ten hours. The half-life varies between people depending on lots of other factors such as age,

body weight, pregnancy status, smoking status, medication and also the health of your liver.

So the coffee you're having later in the day could very well be affecting your sleep at night.

So in general, I would advise having a caffeine cutoff time at around midday or at least eight hours before you go to bed. But back to

your question. Can exercise speed this process up? So one paper looking at the effects of exercise

...