As humans, we spend roughly a third of our lives asleep. When there is natural light, we wake up, and at night we feel drowsy. This phenomenon is called the body's day and night cycle, or the circadian rhythm. circadian rhythms control how we sleep, and when we wake up, they are managed by the rise in fall of certain chemicals, such as the hormones, melatonin and cortisol, and the neurotransmitter adenosine. Melatonin, for example, is sensitive to light, increases with darkness, and helps us to settle into a deep sleep. Cortisol spikes in the middle of the night, and causes REM sleep, the phase where we dream, and then wakes us up. Adenosine builds up during the day, gradually reduces our alertness, and eventually makes us feel tired. Over 24 hours, with someone sleeping from 10pm to 7am, the changes in melatonin, cortisol, and adenosine typically look like this. Now while people all have different circadian rhythms, known as different chronotypes, the rhythm itself shifts as we grow up and age. Children are typically born with an early circadian rhythm, but when they enter puberty, their melatonin and cortisol kick in later, and as a result, their bodies react less to light and get sleepy about 2 hours later. They enter into what's known as a delayed sleep phase. And since most schools don't adapt to this change, teenagers end up sleeping less during weekdays and more during weekends, a phenomenon known as social jet lag, and this leads to all kinds of problems. When teenagers lack rest, their minds have trouble paying attention and absorbing new knowledge, and they often cannot remember what they have learned during the day. This is because sleep is essential for consolidating our experiences into our long-term memory store. Teenagers naturally go through a period of lots of new sensations and often face an identity crisis. Now, people who sleep less naturally also have less REM sleep, the phase where we dream, and it helps us regulate emotions. Those who don't get enough of that might get extra sensitive, moody, or fail to understand their own feelings. A lack of rest leads to more stress during the day, which in turn can mean insomnia at night, a vicious cycle that can hit adolescents who are naturally more tense. In the best case, the stress impairs their creativity. At worst, it leads to anxiety or addictions. This may be one reason why research shows that teenage suicide rates go up when a new school year starts. Sleep also regulates the hormones responsible for our appetite, and so when teenagers don't rest enough at night, they tend to eat more during the day. The result can be obesity or eating disorders. The solution could be simple, adapt social norms to biological facts, and allow teenagers to sleep according to their natural circadian rhythm, and some schools have done just that. In the fall of 2015, Seattle Public Schools announced that it would change the school starting time from 7.50am to 8.45 to adapt to the circadian rhythm of teenagers. The result? People's gained on average over 30 minutes of sleep each night. Later, they displayed higher concentration levels, reported better well-being, and increased motivation to learn. What do you think? Should high schools adapt to the biological rhythm of the people who go there? How is your sleep cycle evolved and impacted your life? Tell us your thoughts and experiences in the comments below. If you liked this video and want us to make more, subscribe to our channel or supporters on Patreon.