signs you're exhausted

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You can drink coffee just before bed and still pass right out.
Some of us are more sensitive to caffeine than others, but no one is completely immune to the stimulant's effects, says Michael Breus, PhD, a clinical psychologist and the author of The Sleep Doctor's Diet Plan: Lose Weight through Better Sleep. The EEG machines that show increased brain activity post-coffee don't lie—but some people swear that caffeine doesn't affect them. What's probably happening with these caffeine denialists, says Breus, is that they're so exhausted that their levels of calm-inducing neurotransmitters are very high. These neurotransmitters help override caffeine's effects, or they set to work on a different area of the brain to kick-start the sleep process—so the caffeine is working, but the coffee drinkers aren't reacting to it. In other words, they're not superhuman; they're just super tired.