Your circadian system wants to wake up at a certain solar time.
People love to talk about the circadian system, because it seems like the most obvious thing in the world — the sun comes up and goes down, and clearly this has some effect on our biology, since we sleep mainly at night.
Often the most interesting things to study are the obvious ones, because often these obvious things are quite a bit more complex once you look under the surface.
The question I want to talk about is this one: what makes someone a night owl and another person an early bird? Circadian biology has an interesting explanation for that question.
Cells have their own idea about how long a day is
From the 2017 Nobel prize, we learned that circadian rhythms happen in many cells in our body — and these cells will keep time even if you take away the daily light and dark cues. The circadian system exists to predict the day, and in a way, the body works best when its predictions are right — no eating at midnight or bright lights at 2AM, because your body already decided those times are at night.
But even the word “circa-dian” means “around a day”, not exactly a day. This means it is rare to find someone with an internal clock that keeps perfect time with the outside world.
Usually this “guess” your cells make about day length is wrong, but only a little bit wrong. If your body thinks the day is 24.3 hours, then you would expect to wake up later and later from day to day. That would happen, and you’d wake up later and later, except for one thing: our bodies also synchronize to light, so that each day your body looks for bright light and corrects the estimate of how long the day is, and then you’re all set: it’s back to 24 hours.
We’re all slightly off in different directions — almost none of us can sit in a dark room and stay on a 24-hour schedule. But we can fix it by seeing light every day, and this works so easily most of us don’t think about it.
Differences between early types and late types
Now the trouble is that this “correction” isn’t that simple, and so you can see that it’s happening. Here’s how.
First, the light that you see in the morning makes your clock set itself earlier, and the light you see at night makes your clock run later. So if those two are balanced (you’re seeing light in the morning and the evening) that means the system predicted the day perfectly and it doesn’t have to make any adjustment.
But let’s think about a person who has a rhythm less than 24 hours, and another who’s a little bit longer than 24 hours (which it turns out, is a majority of us — the average is around 24.2).
The person with a short “day” would start to drift earlier, day by day, if they didn’t have any light cues, right? And when they start drifting, they wake up in the dark, so relative to their internal clock, they will see more light in their own nighttime (because they wake up earlier, the sun stays up later for them). In effect, an early bird will see light at night, and this will push their clocks a little later each day (more light at night than in the morning) and they’ll find a stable way to live a 24 hour day.
In contrast, a night owl will wake up later and later, and then subjectively, that person’s body will see bright light right after they wake up, followed by a relatively early sunset, because their evening will be spent in the dark when they stay up late. Meaning, this person will reach a steady state of 24 hours just by waking up late, seeing bright light when they wake up and missing some at night. In both cases, the difference between daytime and nighttime light matches how far their internal clock is from 24 hours.
We can observe what circadian scientists call “intrinsic period” by watching when someone naturally wakes up, without an alarm clock. Early birds have fast clocks that think a day is shorter than 24h, and night owls have slow ones that run longer than 24 hours.
In the real world, the way this system behaves is that people with a long circadian day — the night owls — end up “looking for” a day that starts with bright light and ends with an early sunset — so they stay up late in order to make sure this is what their body sees.
The early birds “pattern match” for lower light in the morning and extra light at night, and this is why they wake up early, which helps them sleep the best.
But wait, what if people don’t get to pick when they wake up?
If everyone could sleep as long as they wanted, this would be fine, and we’d just see that there’s a difference between people. But where it gets complicated is, well, the other clocks that we keep on our walls. And around 80% of us wake up with an alarm clock, which should be reasonably good evidence that we ask people to get up too early.
When we all try to wake up at the same time for work or school, night owls lose out on sleep the most. And late business dinners for the early birds? Well, they don’t happen every day.
Night owls have to wake up too early
Night owls who wake up with an alarm clock wake up earlier than they would like. Even though they are waking up later than everyone else, they would sleep even later still. So relative to their “ideal” day they are waking up much too early. Their internal clocks are now later than their sleep timing, and this means they don’t sleep as well — they may have trouble falling asleep when they need to, for instance.
Early birds stay up too late
The reverse is true for early birds.
Think of a person who gets invited to a business dinner at 9PM, even though, normally, they would go to bed at 9:30. They go, and stay up late to be at the dinner, but their internal clock wakes them promptly at 4:30 the next morning, anyway.
So, they have stayed up too late for their body. And even though they wake up early, they actually sleep in, to the last minute their bodies will let them sleep. Otherwise we’d see them sleeping later every morning.
The confusing arithmetic of clocks vs. the sun
As I’ve talked to people about permanent DST vs ST, many people get it mixed up. And I have a theory about why this is, and it’s mainly because our identity about whether we’re a night owl or an early bird is about what time it is that we sleep. So when we change the clocks, we can’t reason about what will happen.
I’ve found that people don’t know that night owls biologically “look for” early sunsets, and that early birds “look for” late ones — people often think it’s the opposite. Why wouldn’t a night owl want more light at night? But if you always see through the eye of your own cells, your own clocks, you will find a different answer.
If you think “being a night owl” means you wake up at 10AM and stay up until 2AM, no matter what, when someone asks if you want more light at night, you might think, oh that’s great! More light when I’m awake! But the clocks in your body will say, oh sure, let’s stay up even later, and then it will be way harder to wake up on time.
So while a lot of people prefer a clock change that seems like it would create “more light when I’m awake”, it often works in reverse. Clock changes often make it harder to keep your current schedule, because you can’t wake up as early as you’re required to.
It’s all trying for the same thing: to make the day line up to 24 hours, and the solar clock matters a lot more than the one on the wall.