0:00:07.245,0:00:11.543 Over 100,000 metric tons[br]of caffeine are consumed 0:00:11.543,0:00:13.812 around the world every year. 0:00:13.812,0:00:18.034 That's equivalent to the weight[br]of 14 Eiffel Towers. 0:00:18.034,0:00:21.344 Most of this caffeine is consumed[br]in coffee and tea, 0:00:21.344,0:00:23.793 but it's also ingested in some sodas, 0:00:23.793,0:00:24.667 chocolate, 0:00:24.667,0:00:26.244 caffeine pills, 0:00:26.244,0:00:29.264 and even beverages labeled decaf. 0:00:29.264,0:00:32.991 Caffeine helps us feel alert,[br]focused, happy, and energetic, 0:00:32.991,0:00:35.982 even if we haven't had enough sleep. 0:00:35.982,0:00:40.410 But it can also raise our blood pressure,[br]and make us feel anxious. 0:00:40.410,0:00:43.153 It's the world most widely used drug. 0:00:43.153,0:00:46.612 So how does it keep us awake? 0:00:46.612,0:00:50.892 Caffeine evolved in plants where[br]it serves a few purposes. 0:00:50.892,0:00:55.474 In high doses, as it's found in the leaves[br]and seeds of certain species, 0:00:55.474,0:00:57.784 it's toxic to insects. 0:00:57.784,0:01:01.672 But when they consume it in lower doses,[br]as it's found in nectar, 0:01:01.672,0:01:05.924 it can actually help them remember[br]and revisit flowers. 0:01:05.924,0:01:11.403 In the human body, caffeine acts as a[br]stimulant for the central nervous system. 0:01:11.403,0:01:15.747 It keeps us awake by blocking one of[br]the body's key sleep-inducing molecules, 0:01:15.747,0:01:19.402 a substance called adenosine. 0:01:19.402,0:01:22.013 Your body needs a constant supply[br]of energy, 0:01:22.013,0:01:27.314 which it gets by breaking down[br]a high-energy molecule called ATP. 0:01:27.314,0:01:28.254 In the process, 0:01:28.254,0:01:32.794 it liberates adenosine,[br]ATP's chemical backbone. 0:01:32.794,0:01:37.645 Neurons in your brain have receptors[br]perfectly tailored to this molecule. 0:01:37.645,0:01:39.954 When adenosine docks to these receptors, 0:01:39.954,0:01:43.665 it activates a cascade [br]of biochemical reactions 0:01:43.665,0:01:45.926 that cause neurons to fire more sluggishly 0:01:45.926,0:01:50.465 and slow the release of important[br]brain-signaling molecules. 0:01:50.465,0:01:53.265 In other words, you get sleepy. 0:01:53.265,0:01:58.285 Caffeine is what's called an adenosine[br]receptor antagonist. 0:01:58.285,0:02:01.856 That means it derails this process[br]of slowing your neurons down 0:02:01.856,0:02:04.385 by blocking adenosine receptors. 0:02:04.385,0:02:08.627 Caffeine and adenosine have a similar[br]molecular structure, 0:02:08.627,0:02:12.456 close enough that caffeine can wedge into[br]the adenosine receptors, 0:02:12.456,0:02:15.266 but not close enough to activate them. 0:02:15.266,0:02:18.855 To summarize, [br]adenosine inhibits your neurons. 0:02:18.855,0:02:24.727 Caffeine inhibits the inhibitor,[br]so it stimulates you. 0:02:24.727,0:02:27.306 Caffeine can also boost positive feelings. 0:02:27.306,0:02:29.855 In some neurons, the adenosine receptors 0:02:29.855,0:02:34.367 are linked to receptors [br]for another molecule called dopamine. 0:02:34.367,0:02:38.570 One of dopamine's roles in the brain[br]is to promote feelings of pleasure. 0:02:38.570,0:02:41.871 When adenosine docks in one of these[br]paired receptors, 0:02:41.871,0:02:45.948 that can make it harder for dopamine[br]to fit in its own spot, 0:02:45.948,0:02:48.758 interrupting its mood-lifting work. 0:02:48.758,0:02:53.338 But when caffeine takes adenosine's place,[br]it doesn't have the same effect, 0:02:53.338,0:02:55.948 and dopamine can slide in. 0:02:55.948,0:03:00.930 There's evidence that caffeine's effects[br]on adenosine and dopamine receptors 0:03:00.930,0:03:05.180 can have long-term benefits, too,[br]reducing the risk of diseases 0:03:05.180,0:03:10.129 like Parkinson's, Alzheimer's,[br]and some types of cancer. 0:03:10.129,0:03:14.998 Caffeine can also ramp up the body's[br]ability to burn fat. 0:03:14.998,0:03:17.510 In fact, some sports organizations think 0:03:17.510,0:03:20.960 that caffeine gives athletes [br]an unfair advantage 0:03:20.960,0:03:24.308 and have placed limits on its consumption. 0:03:24.308,0:03:27.120 From 1972 until 2004, 0:03:27.120,0:03:31.739 Olympic athletes had to stay below[br]a certain blood-caffeine concentration 0:03:31.739,0:03:34.120 to compete. 0:03:34.120,0:03:37.729 Of course, not all of caffeine's effects[br]are so helpful. 0:03:37.729,0:03:40.089 It might make you feel better[br]and more alert, 0:03:40.089,0:03:43.220 but it can also raise your heart rate[br]and blood pressure, 0:03:43.220,0:03:46.410 cause increased urination or diarrhea, 0:03:46.410,0:03:49.953 and contribute to insomnia and anxiety. 0:03:49.953,0:03:52.801 Plus, the foods and beverages[br]caffeine is found in 0:03:52.801,0:03:58.541 have their own impacts on your body[br]that have to be taken into account. 0:03:58.541,0:04:01.571 Your brain can adapt to regular[br]consumption of caffeine. 0:04:01.571,0:04:04.832 If your adenosine receptors [br]are perpetually clogged, 0:04:04.832,0:04:07.720 your body will manufacture extra ones. 0:04:07.720,0:04:09.939 That way, even with caffeine around, 0:04:09.939,0:04:15.252 adenosine can still do its job[br]of signaling the brain to power down. 0:04:15.252,0:04:18.723 That's why you may find you need[br]to consume more and more caffeine 0:04:18.723,0:04:20.853 to feel as alert. 0:04:20.853,0:04:24.473 There are more and more adenosine[br]receptors to block. 0:04:24.473,0:04:26.973 It's also why if you suddenly [br]quit caffeine, 0:04:26.973,0:04:30.194 you may experience [br]an unpleasant withdrawal. 0:04:30.194,0:04:32.493 With plenty of receptors [br]and no competition, 0:04:32.493,0:04:34.853 adenosine can work overtime, 0:04:34.853,0:04:36.473 causing symptoms like headaches, 0:04:36.473,0:04:37.634 tiredness, 0:04:37.634,0:04:40.212 and depressed moods. 0:04:40.212,0:04:44.963 But in a few days, the extra adenosine[br]receptors will disappear, 0:04:44.963,0:04:46.993 your body will readjust, 0:04:46.993,0:04:49.593 and you'll feel just as alert as ever, 0:04:49.593,0:04:53.233 even without an infusion of the world's[br]most popular stimulant.