# Circadian Rhythm We evolved to have different chronotypes (circadian rhythms). Night owls would go to bed around 1 or 2 am, and wake up between 9 and 10 am. Morning larks would go to bed around 9 pm and wake up around 5 am. Therefore, the period when the whole tribe was asleep, and therefore most vulnerable, would have been reduced in half to about 4 hours, possibly increasing our survival chance by 50%. The suprachiasmatic nucleus is the part of the brain that sends the circadian signal to the rest of the body. It uses the hormone melatonin to do this. It begins to be released at dusk from the pineal gland. Melatonin signals other areas of the body to begin generating sleep, but it does not itself generate sleep. When dawn arrives, and light enters the eyes through the eyelids, a brake is applied to the pineal gland to stop the production of melatonin. Melatonin supplements are not useful for inducing sleep unless one is jetlagged. They are also not highly regulated, and studies have found that over-the-counter brands have concentrations from 83 percent less to 478 percent more than the stated amount. For every day that we are jetlagged, the suprachiasmatic nucleus can only adjust by one hour. It's harder to adjust to jetlag when we are travelling East than West. This is because travelling East results in a shorter day, and it's hard to force oneself to go to bed early. But travelling West results in a longer day, and it's easier to force ourselves to stay up. Also, our circadian clock is actually 15 minutes longer than the 24-hour day, so adjusting to a longer day is easier than to a shorter day. # Sleep Pressure A chemical called adenosine is constantly building up in our brain. The longer we are awake, the more adenosine builds up. High concentrations of adenosine simultaneously turn down the activity of wake-promoting regions and turns up the activity of sleep-inducing regions. The effect is called sleep pressure, and this is another reason besides the circadian rhythm which causes us to get sleepy. Most people feel an irresistible urge to sleep after 12 to 16 hours. Caffeine artificially mutes the signal from adenosine. Caffeine is the second most traded commodity after oil. > The consumption of caffeine represents one of the longest and largest unsupervised drug studies ever conducted on the human race, perhaps rivaled only by alcohol, and it continues to this day. Caffeine competes with adenosine for uptake by adenosine receptors in the brain. When caffeine binds to these receptors, it doesn't send the sleepiness signal, and it tricks the brain into feeling alert. Caffeine has an average half-life of 5-7 hours.