According to the National Institutes of Health, “Circadian rhythms are physical, mental, and behavioral changes that follow a roughly 24-hour cycle, responding primarily to light and darkness in an organism’s environment,” and they are found in most living organisms. In simpler terms, they’re internal and external factors that compel humans, plants, animals, or microorganisms to carry out their biological processes on a daily schedule. For example, plants that open and close their leaves or petals by day and night, respectively, are responding to environmental stimuli such as light or temperature but on a schedule of sorts driven by their inner circadian clock.
In our everyday lives, humans most commonly experience circadian rhythms as the drives that compel us to sleep and eat on a regular schedule, and there are disruptions of this schedule that can occur in instances such as jet lag. Think about it: If your sleep schedule was wholly independent of time on a 24-hour cycle, wouldn’t you be able to sleep at any time if you were sufficiently tired, even through the whole day? Obviously, many people do this occasionally or have to do so because of night shifts at work or other factors, but the commonly accepted, regular pattern is to sleep at night and to stay awake during the day. This is partly because of changes in lighting, but it’s also partly because that’s the schedule we’re coordinated to have due to production of a hormone called melatonin by a cluster of nerve cells in the brain called the suprachiasmatic nucleus (SCN). The SCN is the “body clock” that controls circadian rhythms and releases hormones that compel us to sleep and eat.
Circadian rhythms are then the reason for a number of things relating to your health. They’re why sleeping on a regular schedule is important since a regular sleep schedule corresponds to regular brain wave activity, cell regeneration and healing, and hormone production. Disrupted sleep schedules can contribute to heightened risk for obesity, diabetes, and a number of mental disorders, including depression, bipolar disorder, and seasonal affective disorder.
They control how awake you are at any given time, such as the groggy feeling you might feel in the morning after waking up or the rush of energy you sometimes get in the afternoon or evening. Some studies suggest that the circadian rhythms of young adults operate on a slightly different schedule, which makes it difficult to wake up and go to sleep as early as would be ideal. This has led to a push for later school start times because teenagers and young adults may not be able to wake up at peak condition and get to school as early as most high schools currently start.
Another set of factors playing into your sleep health are your sleep cycles, or “characteristic patterns of activity throughout each period of sleep” that last approximately 90 minutes each. To grossly simplify the whole process, you switch back and forth between REM sleep (rapid eye movement sleep, in which people dream much more frequently than in other periods) and NREM sleep (non-rapid eye movement sleep). The reasons for these patterns are unknown, but they’re believed to exist in this pattern in order to maximize physical and mental recovery and recall of the day’s memories. Five to 6 of these cycles is considered an ideal number, and you’re more likely to wake up alert if you wake up at the end or beginning of a cycle rather than in the middle. There are even calculators, like this one, that allow you to figure out the exact best time for you to wake up in the morning based on the time you go to sleep.
These patterns can, however, be changed, as illustrated by the fact that your sleep habits eventually adjust to whatever time zone you’re in after a brief period of jet lag. Your sleep history and any sleep irregularities you have eventually produce a change in the length, sequence, or intensity of your REM or NREM sleep cycles. As much as it would probably be ideal to follow our natural circadian rhythms as much as possible and to conform to the exact amount and timing of our sleep cycles, that’s just not possible in the busy world we live in — especially for teenagers and young adults whose classes begin at 8 AM or earlier and who don’t have the defined 9-to-5 schedule that many adults do.
In fact, because of how easily these cycles can adapt to fit a person’s sleep history and how little scientists have actually confirmed about how or why they exist the way they do, circadian rhythms and sleep cycles may be less important than we make them out to be. In my opinion (granted, a rather uninformed one, as I’m a 16-year-old high school student and not a licensed medical professional), a knowledge of them can help you better understand the functioning of your own body and best figure out what sleep habits work for you.
From my personal experience, most people don’t fit these cycles in the way science suggests they ideally should. My mom sleeps what’s probably a healthy length of time for an adult her age, and in a fairly regular pattern if our dogs don’t disrupt her sleeping; but, without fail, she wakes up to read the New York Times on her phone in the middle of the night, every single night. My friend is on a cycle determined mostly by her intensive, full-IB school schedule, in which she goes to sleep very late in the evening, sleeps for a couple hours, wakes up in the middle of the night to work on schoolwork or study, and goes back to sleep for a couple hours before she needs to wake up again to get ready for school. Both of them are on what’s called a biphasic sleep cycle. Some of my friends just need a straight 8 to 9 hours, otherwise they can’t function at all the next day in class.
Personally, I am one of the worst people to write an article on the subject of sleep schedules since my sleep is highly irregular and would probably be considered as highly unhealthy by most scientists. I pretty much sleep when I can and take a lot of naps. The positive here is that I can fall asleep almost instantly, whether at nighttime or in the middle of the day if I want to, and I don’t fall asleep unintentionally. The negative is that I average much fewer hours of sleep a night than is considered healthy for someone my age and that I have really no schedule at all. Sometimes I stay up late, sometimes I wake up extremely early, and sometimes I take naps and do homework in 20-minute intervals in the middle of the night. There’s no pattern. A few weeks ago I stayed up for 37 hours straight and made up for it with 11 hours of sleep plus a 90 minute nap the next day (which I would not recommend, by the way). It’s really different than the rigid, “7.5 hours every night or else” schedule I generally followed a year ago, and it’s probably not healthy in a traditional sense, but it’s worked pretty well for me so far.
The point of all of this? No, I wouldn’t recommend picking up my sleep schedule, and I wouldn’t consider myself a role model for sleep habits in any sense. I wouldn’t recommend trying out any polyphasic sleep cycles you happen to find on the Internet because they only work for some people on a very rigid, difficult-to-accommodate schedule. I wouldn’t even recommend trying to follow what science says your circadian rhythms and sleep cycles should be like to a tee because that’s probably just as impractical for your daily life as a complicated one would be.
What would I recommend, then? Find something that works for you. Some people naturally need more sleep. Some people naturally need sleep at different times and in different patterns or need to adjust their sleep to a certain schedule. Because of genetic differences between every individual, let alone the lack of solidified knowledge on the exact nature of components of sleep, there’s no way that science can tell you exactly how or when to sleep. The best thing you can do for yourself and your health — in all aspects of your physical well-being, including nutrition and exercise — is to try out different things until you find something that makes you feel optimally healthy.