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Published 17:54 12 Feb 2015 GMT
Updated 17:57 12 Feb 2015 GMT

Cortisol is the hormone that controls both stress levels and blood sugar levels and Miller believes drinking coffee before or during these hours can lead to "desensitisation".
This basically means that if you drink coffee before 9:30am, you'll need more than one cup before the caffeine kicks in.
We're definitely going to trial this theory and see if it makes any difference to our coffee cravings...Explore more on these topics: