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Yes, there really are "early birds" and "night owls" and no, you can't simply train yourself to do better at a different time of day using nothing but willpower and habit. While night owls may grumble in the morning when they have to get up "way too early" and early birds may prefer their warm nests to late-night events, most people don't really spend a lot of time thinking about what makes their natural clock the way it is. That's a shame, because the science of night owls vs early birds is actually awfully fascinating. What's more, your circadian rhythm says a lot about your personality as well as your mental and physical health.
Your Internal Clock
Your personal internal clock, your circadian rhythm, roughly coincides with a calendar day — it responds to the Earth's light-day cycle. Interestingly enough, this holds true not just for humans but also for animals, plants, and even many microbes! Over the course of a day, your body goes through physical, mental and behavioral changes. When we say a circadian rhythm lasts roughly 24 hours, that last bit is important however: while some people have slightly shorter cycles, others have somewhat longer cycles.
In charge of it all is your very own "biological clock". This is controlled by a center in the brain called the suprachiasmatic nucleus or SCN, in the hypothalamus. The SCN in turn directs molecules all over the body, and is in charge of your circadian rhythm, including the hormone melatonin, which gets you ready for sleep. Besides regulating your sleep-wake cycles, the circadian rhythm influences the release of hormones, body temperature and other physical functions. If it isn't working properly, you may end up with sleep disorders like insomnia, but problems with your circadian rhythm have also been linked with diabetes, obesity, depression, and seasonal affective disorder, which makes people feel depressed during winter.
Your internal clock affects your life in very important ways, in other words. Considering the fact that most people don't spend a lot of time thinking about what drives their natural rhythm, that is truly fascinating. The question is, are you a slave to your circadian rhythm or is there anything you can do to alter it?
Can Your Circadian Rhythm Change With Time?
Yes, your circadian rhythm can indeed change with time, and it usually does a bit. While many young kids are naturally early risers (as anyone with toddlers who wake them at 6 am knows!), teens are renowned for their tendency to sleep in, or over-sleep. They also stay up later, obviously, while small children turn in early. Many people find that their sleeping patterns gradually drive them to go to bed earlier and rise earlier as they wonder out of adolescence into adulthood and middle age.