In the grand scheme of life, timing is everything — from the right moment to broach a promotion with your boss to the exact date you pop the question to your significant other. Our society is so attuned to time that every activity in our daily life heeds the ticking of the clock on our smartphone, laptop, car dashboard, and so forth. The same goes for sleep.
But your specific bedtime may not echo that of others. Perhaps you're a morning person who prefers an early-to-bed, early-to-rise sleep schedule. Meanwhile, your teenage son only hits the sack at three in the morning but insists because he sleeps until mid-morning that’s fine. This gets you thinking, “Does it matter what time you sleep?”
Here’s the long and short of it: Your sleep time does matter — if you want to keep your sleep debt low and your circadian rhythm aligned. With that said, there's one caveat to look out for. Keep reading to find out more.
Does it matter what time you sleep? To answer the question satisfactorily, the Two Laws of Sleep (based on the two-process model of sleep regulation first established by Alexander Borbély in the 1980s) is key to understanding sleep and its ideal timing.
Sleep homeostasis refers to the daily buildup and nighttime depletion of sleep pressure. This regulation of sleeping and waking is mediated by the drowsiness-inducing compound adenosine.
Over a roughly 24-hr period, adenosine waxes and wanes in your brain to help you fall asleep at night and stay asleep till morning. While you sleep, your brain purges accumulated adenosine to reset the sleep homeostat.
But what if you go to bed later than usual and don't meet your sleep need? For reference, that's the genetically determined amount of sleep your body needs to feel and function at its best the next day (we don’t all need the much touted 8 hours; most of us fall on a spectrum somewhere between 7.5 and 9).
A shorter-than-your-biological-need sleep duration inevitably means less time for the adenosine to fully dissipate before you wake. Consequently, you wake up feeling drowsier than you should, courtesy of the leftover adenosine in your system.
Not meeting your sleep need means you rack up sleep debt, the amount of sleep you've missed out on in the past 14 days relative to your sleep need. Because sleep debt dampens your daytime energy potential (think: flatter energy peaks and deeper energy dips), it does matter what time you sleep, as you'll see later. To complicate matters, everyone's sleep need is genetically unique to them, so it can be difficult to track whether you've had enough sleep on a nightly basis.
That's why a tool like the RISE app is profoundly useful in helping you determine your sleep need and then your sleep debt. It uses sleep-science-based models and the past 365 nights of sleep data tracked by your phone to learn your unique sleep biology and calculate your sleep need in hours and minutes. From there, RISE helps you keep on top of your sleep debt, which you can view on the Sleep screen.
Ideally, your circadian rhythm — the internal body clock that dictates your energy peaks and dips over a roughly 24-hour cycle — works synergistically in relation to sleep homeostasis to govern your sleep-wake cycle. Its end goal is to help you get the naturalistic, healthy sleep your body needs. Not only does your internal clock guide the daily fluctuations in your energy levels, but it also determines your ideal sleep and wake times to give you the best chance of meeting your sleep need.
Before you assume that everyone’s ideal sleep-wake schedule is one and the same, your chronotype dictates your biological bedtime and rise time in relation to your circadian rhythm (amongst other biological processes like eating and exercising). Chronotypes are mainly classified into early birds, night owls, and intermediate types (somewhere in between). To learn more about chronotypes and how you can make the most of yours, check out our in-depth guide on the subject.
To help you stay on the same wavelength as your circadian rhythm, the RISE app tells you the best windows during which to sleep and wake, as well as the times of your energy peaks and dips on your Energy Schedule. It follows a predictable pattern like this:
Keep in mind that the timing and length of the circadian activities vary from individual to individual. To illustrate, research points out that the internal body clock "programs a longer biological night in long sleepers than in short sleepers."
So, does it matter what time you sleep? For the most part, the answer is yes. The only exception is when you have high sleep debt, as you’ll see below.
While minimal sleep loss is relatively normal and acceptable, more than five hours of sleep debt means you’re more sleep-deprived than is good for you. Perhaps you often engage in sleep procrastination (like Netflix binge-watching and Instagram scrolling). Or maybe your long to-do list makes you think you can survive on less sleep than recommended. (Spoiler alert: You can't.)
If that sounds like you, prioritize catching up on sleep when you can rather than fixate on your ideal sleep timing. There are four ways you can pay down sleep debt with only minimal circadian misalignment (if any):
Pro tip: The Energy Schedule in the RISE app can help you structure your day in a way that gives you more shuteye to lower your sleep debt.
Even though we're advocates of bringing down sleep debt, and whenever you can if you’re truly sleep deprived, there are a few scenarios when it does matter what time you sleep.
For instance, falling asleep while driving is a very real yet injurious scenario that can happen to any of us. It usually occurs when your sleep pressure is so high during the day that you unknowingly succumb to microsleeps, a phenomenon in which your brain goes offline while your eyelids are partially or fully closed.
The danger of microsleeping is that it can happen anywhere, anytime — at your work desk, in the grocery store, or during your yoga class. It's a no-brainer then that nodding off at the wrong time can cost you your job, your social relationships, and even your life (as well as endangering those around you).
Not heeding your body’s biological inclination toward sleep and wake times that match your chronotype is not the only thing to look out for when determining what time to sleep is best for you. Irregular sleep patterns can rattle your circadian rhythm, leading to circadian misalignment.
Most commonly, this takes the form of social jetlag, in which your social and biological clocks are out of sync. Case in point: You sleep and get up late on the weekends (or your days off) and revert back to the 6 a.m. wake-up calls when your work week resumes. Consequently, you struggle with more intense sleep inertia when you awake on Monday morning, with excessive daytime drowsiness dogging your footsteps throughout the day.
For a fair percentage of the population that works the night shift, circadian misalignment also manifests as shift work disorder (SWD). Trying to be conscious and productive during the graveyard shift when your brain is primed to be asleep is trying in and of itself. Your body pays for it in the form of an out-of-whack body clock. A 2018 study noted that just four days of night shift is enough to steer your circadian rhythm off-course.
Sleep deprivation — and the accompanying daytime sleepiness, reduced attention span, slower reaction time, increased moodiness, poorer immunity, and greater risk of accidents — is often the most direct and immediate repercussion shift workers struggle with. But the train wreck of circadian misalignment doesn't stop there.
A recent small-scale study analyzed what happens when you work a night shift or day shift schedule for at least five years. Night shift workers had deregulated clock genes (which governed the internal clock), suggesting circadian misalignment. Compared to day shift workers, night shift workers exhibited more worrying health symptoms, like higher blood sugar levels and greater weight gain. This means they are now at a greater risk of developing metabolic diseases like obesity and Type 2 diabetes.
Aside from sleep problems, SWD is also closely associated with a greater prevalence of heart disease, mental health issues like depression, and even certain types of cancer (for example, breast cancer and prostate cancer). According to the Centers for Disease Control and Prevention (CDC), recent updates by the The National Toxicology Program (NTP) and the International Agency for Research on Cancer (IARC) surmised that night shift work is “probably carcinogenic to humans,” meaning that it functions as a cancer-causing activity.
Healthy sleep habits are the secret formula to less than five hours of sleep debt. But if your sleep hygiene isn't tied to your circadian rhythm, you're not immune to an off-kilter body clock. You're just as vulnerable to circadian misalignment as those suffering from a lack of sleep. Unfortunately, even if you have little sleep debt, circadian misalignment is enough to give rise to suboptimal energy levels.
That's why the answer to "Does it matter what time you sleep when you have minimal sleep debt?" is a clear, resounding yes. The next question to ask yourself is, "When's the best time for you to go to bed to keep your circadian rhythm aligned?" Two words: Melatonin Window.
Two to three hours before your biological bedtime, your brain starts secreting melatonin in earnest. Scientifically known as the dim light melatonin onset (DLMO), it marks the start of your Melatonin Window, when your body produces peak levels of melatonin. But why do we place so much emphasis on your Melatonin Window in the context of circadian alignment?
The scientific literature explains that DLMO (and, by extension, your Melatonin Window) is a vital time cue that regulates your circadian rhythm. For that reason, your Melatonin Window is a "measure of circadian alignment."
In the name of better sleep and a well-tuned body clock, turn in within this window of time. If you're trying to pay down sleep debt, go to bed at the start of your Melatonin Window to give yourself extra snooze time. With the rare exception of paying down colossal amounts of sleep debt (in which case, sleep as early as you can per our earlier recommendation), avoid an earlier bedtime of more than an hour before your Melatonin Window. It can be counterproductive and even frustrating to lay in bed when your body isn't prepared to sleep yet.
For the most part, it does matter what time you sleep to keep your circadian rhythm aligned for minimal sleep deprivation and optimal energy levels. The only caveat is when you’re saddled with significant sleep debt. In which case, prioritize getting the sleep you need without tripping up your internal clock, if possible.
Because it can be confusing to know the specific times you should sleep relative to your circadian rhythm, the RISE app is infinitely helpful in taking the guesswork out of your ideal sleep timing. This is especially so if you identify as a poor sleeper or have an irregular sleep schedule due to social and work obligations. Our app's Smart Schedule feature comes in handy here. It suggests a bedtime that gently trains your body to meet your sleep need and still be aligned.
The best part about RISE is that it's all taken care of for you in the app. All you have to do is turn on your notifications for the habits on your Energy Schedule — think: "Peaks & Dips," "Evening Routine," and "Melatonin Window," to name a few. This way, you'll receive well-timed reminders pegged to your unique chronobiology for better sleep and better days.
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