This is a guest post written by Daniel Pardi.
As a sleep researcher and sleep educator, the most common question I get is, “How do I know how much sleep I need?” It’s a tricky question because the benefits of sleep are only partially determined by sleep duration. When people ask this question, what they really mean is, “What are the things I need to do in order for me to get all the physiological and behavioral benefits of a perfectly managed sleep-wake practice?” This article will help you answer that question, and one component of that question is understanding how much time you need to spend in bed per night. First, we need to review important fundamentals of sleep and wake regulation, otherwise the guidance here won’t make much sense.
You’ve heard of the importance of personalizing your diet. But what about your sleep?
While we don’t know that a single purpose for sleep exists, we do know that many important things take place during that period of time between when you shut your eyes at night and when you open them again in the morning. A major breakthrough in the field of sleep research took place in 1982 when Dr. Alexander Borbely, Emeritus Professor at the University of Zurich, published an article in the journal Human Neurobiology describing a concept of how sleep and wake are controlled over a 24-hour period. He dubbed his idea the 2-Process Model of Sleep Regulation, and building upon the evidence available at that time, he changed the way the world understood how sleep and wake are regulated. In Dr. Borbely’s model, he described that the regulation of these states is based primarily upon the combination of two different processes occurring simultaneously. To understand how to get the sleep you need, let’s explore those two processes further.
Process 1 – sleep pressure
The first process of the professor’s 2-Process Model is sleep pressure. From the moment you wake up in the morning, a pressure for sleep mounts continuously until you sleep again. At night, the built-up sleep pressure helps you 1) fall asleep, and 2) stay asleep until the next morning. During sleep, this pressure decreases, and after a night of complete sleep, you start the next day with low sleep pressure. Later in the article, I’ll talk about the concept of “sleep debt,” which happens when you don’t wear down all your sleep pressure before starting the next day.
If sleep pressure builds from morning to night, why don’t we become increasingly sleepy from the moment we wake up? That’s where the second process of the 2-Process Model comes into play.
Process 2 – wake drive
This second process is called “wake drive.” You can think of it as the result of multiple brain systems working together—a wake network—to produce an alerting signal that increases in intensity to counteract mounting sleep pressure. The result? While sleep pressure builds from morning to night, wake drive also builds from morning to night, and your alertness stays relatively even across the day (with some within-day oscillations). This process raises another question: If wake drive is high during the night, how do we fall asleep? To understand the answer, we need to discuss how sleep pressure and wake drive differ.
Unlike sleep pressure, which is determined by time awake and time asleep, wake drive is controlled by a 24-hour rhythm, like an oscillating sine wave (i.e., a repeating wave-pattern with a high point and low point). For instance, assume someone typically wakes up at 7 a.m. and goes to bed at 11 p.m. and that his or her wake rhythm is stable and fixed to this time frame. Under these conditions, their wake drive initiates at 7 a.m., builds all day to counteract mounting sleep pressure, reaches its peak at 10 p.m., and plummets at 11 p.m., descending to its lowest point across a 24-hour period. At 7 a.m. the next morning, it begins to build back up all over again.
This rhythm repeats day after day—you awake at the same time and feel sleepy at the same time, on a daily basis. Importantly, the timing of the rhythm can be shifted forward or backwards. In a second example, imagine the wake rhythm is shifted two hours forward: wake drive starts at 9 a.m., builds all day, reaches its peak at midnight, plummets at 1 a.m. to its lowest level, and then builds back up again around 9 a.m. the next morning. It is easy to see that the timing of this rhythm is crucial for daytime alertness and quality night-time sleep. Let’s explore what sets the timing of our daily wake rhythm.
Imagine you live in San Francisco and travel halfway around the globe to Rome, Italy. When you arrive, your internal rhythms are still synced to San Francisco. At the beginning of your trip, you’re awake at night when everyone in Italy is sleeping and sleepy during the day when everyone else is fully awake. But slowly, over the course of multiple days, your body starts to adjust to this new time zone. What signal is the body responding to in order to adjust the timing of its internal rhythms to be in sync with the environment? The primary signal that sets our 24-hour body rhythms, including the timing of our wake rhythm, is light entering the eye.
There is an incredible amount of information regarding this topic, but I’ll get to the bottom line. To set the timing of your wake rhythm, your brain wants to coordinate the timing of your wake signal to daytime. It does this by measuring the intensity and hue of light entering the eye. Indoor light is far less intense than outdoor light and, more than ever in human history, we spend much of our day indoors. So, bright daylight anchors your wake rhythm, and when you don’t get enough daylight, your wake rhythm shifts forward. This effect is compounded by getting too much artificial light at night. Essentially, by having light enter the eye at night when the brain should be getting exposure to darkness, we are again telling the brain that it should shift the timing of the wake rhythm forward. So, living in the modern world causes a double-whammy—too little light during the day and too much light at night—shifting our wake rhythm forward.
What is one implication of having your wake rhythm shifted forward? Let’s say you need to get up at 7 a.m. and want to go to bed by 11 p.m. Remember, you start to build sleep pressure as soon as you wake up. Not a problem if your wake rhythm starts at 7 a.m. too. Everything works really, really well when these systems are coordinated. But, when your wake rhythm is shifted forward, say, to 9 a.m., now you have a mismatch between sleep pressure and wake drive. There is a gap between the intensity of these two counteracting forces, and that gap is maintained all day.
What do you experience when this happens? Day-long sleepiness and impaired alertness (and a host of other cognitive issues). But it doesn’t end there. Let’s look at what happens to sleep at night. In this scenario, you want to go to sleep at 11 p.m., and you’ve been sleepy all day, but since your wake rhythm is shifted forward, it’s at its most powerful at the time you want to go to bed. This can cause insomnia or it can make the first phase of sleep shallow. Incidentally, the first phase of sleep should be the deepest, and this depth of sleep corresponds with the fastest reduction of sleep pressure. If you don’t wear down sleep pressure efficiently, you have to sleep longer to do so. But if you don’t sleep longer, instead waking by an alarm as most people do, you wake the next day with “sleep debt,” which, as I described earlier, means you carry some of yesterday’s unresolved sleep pressure with you into tomorrow. So, a mistimed wake rhythm can either shorten your sleep by making it harder to go to bed at night or disrupt the depth of your sleep, either of which will cause sleep debt. What this means is that seven hours of sleep will feel more like six the next day. Conversely, you could get nine hours of sleep (which might be a lot for you) and still not feel well rested.
You can see why it’s difficult to directly answer the question, “How much sleep do I need?” Sleep quality is not entirely dependent upon sleep duration. Sleep quality could be disrupted by a wake rhythm that is overly active in the first part of the night, making it hard to nod off, or making the first sleep phase, the deepest sleep phase, shallower. The big lesson you should learn here is to anchor your wake rhythm by maintaining smart light habits throughout the day, evening, and night. To do this, get at least 30 minutes of outdoor light exposure during the day. In the evening, dim all lights and reduce blue or full-spectrum light in your environment. At night, while you sleep, maintain a very dark room.
Determining your ideal sleep duration
Given that the timing of your wake rhythm can shift, and that this shift can then influence sleep duration ability and needs, sleep duration is an important part of the puzzle. Let’s explore the topic, assuming your wake rhythm is well-timed and stable. Scientists have taken many approaches to try to answer the question, “How much sleep does one need?” A series of early studies put subjects in a stimulation-free environment for 14 to 24 hours per day and then monitored daily sleep quantity. Each day for the first two days, subjects slept up to 12 to 20 hours! Then, on subsequent days, daily sleep times reduced until each subject arrived at a consistent daily sleep amount. Most people settled at an average of eight hours per night, although some settled closer to nine and others closer to seven. Once each subject settled at his or her stable daily average, after they had the chance to wear down additional sleep pressure, this average likely approximated their actual daily sleep requirement.
One way to interpret these findings is to assume that subjects entered these studies with extra sleep debt that built over time and could be attributed to less-than-optimal nightly sleep before they entered the study. When given way-more-than-normal time for sleep in the study, this built-up pressure caused long sleep periods in the first few days, and decreased over time to settle at a unique average for each person. Interestingly, although the volunteers reported feeling perfectly normal when they entered the study, after they reduced this extra sleep pressure, they had notable improvements in mood and energy levels.
More recently, sleep researchers Siobhan Banks, David Dinges, and others have been exploring this topic further. In some of their studies, they restrict sleep time during the week to four or five hours per night, then give subjects recovery sleep for several days in a row. Recovery sleep means that the subjects were allowed to sleep as long as they could, only waking up on their own volition and not by external means. What they found is quite telling. Not only does performance on mental tasks increasingly degrade each day during night after night of insufficient sleep, but when finally given a chance to sleep longer, they do—much longer, in fact. It’s common for these subjects to sleep 125 percent of normal, going from eight hours a night at baseline to ten hours a night during recovery nights. But, their sleep debt wasn’t fully worn down by one long slumber. Subjects had increased sleep duration beyond their baseline needs lasting for about four nights. And this occurred after only five nights of sleep deprivation. In real life situations, people will restrict sleep beyond five days fairly regularly. In fact, one-third of the U.S. population reports sleep less than six hours a night on average. I won’t go into the all the associations that have been found between chronic sleep restriction and disease, but my global impression is that the effects of chronic, persistent sleep loss promote a whole-system pathological response in humans. In other words, lots can go wrong and does.
What if I can’t sleep past 7 a.m.?
If you can’t sleep past the time you usually wake up in the morning, does this mean you’re getting all the sleep you need? Well, actually, no. Let’s say you have a fairly strong wake rhythm that starts at 7 a.m. each day. Excellent! But, even if you haven’t worn down all your sleep pressure from the night before, you still may wake up at your usual time. You just might be sleepier than you need to be under this condition. So, one way to evaluate sleep need is to look at how long you can sleep. However, given this potentiality, you might just be experiencing sub-functional mental capacity during the day. In this scenario, try an experiment. Attempt to get complete nightly sleep for three weeks in a row, plus maintaining smart light habits day by day, and then see how you feel. To start, consider how much time you typically spend in bed per night. Tracking sleep manually or with the sleep tracker on Dan’s Plan is very useful for this. One objective measurement to indirectly assess sleep quality is to evaluate reaction time during the day, which is a measure of vigilance. For the first three days of the experiment, at the same time each morning, do 20 tries of a reaction time test at HumanBenchmark.com. Write down your average reaction time across each day, then average those 3 days into one score and file it as “baseline” in a place where you can reference it later.
- Day 1: 269 milliseconds (ms)
- Day 2: 303 ms
- Day 3: 277 ms
Baseline Reaction Time Score (3-Day average): 283 ms
Now that you have an idea of your average time in bed each night, add 30 to 60 minutes to that time by going to bed earlier. Try not to wake by an alarm during this experiment. Do not spend too much effort in evaluating how you feel during this three-week period. Instead, evaluate how you feel after three weeks have passed. The goal of the Dan’s Plan Restorative Sleeper concept is to wake refreshed and feel alert all day. How do you feel? Noticeably different and better? Also, do another three days of morning reaction time tests at humanBenchmark.com and see if your scores differ from your baseline. If you do see improvements, you’ve identified that you benefit by getting more sleep than your previous “normal.” Many people are surprised by how good they feel when they undergo this experiment, and that experiential evidence can motivate you to maintain this propitious nightly bed duration over the long term. If you didn’t sleep any longer and don’t feel much different, then you may be getting all the sleep you need on a regular basis. Either way, you’ve added knowledge to your own sleep practice and can then make corresponding adjustments if need be.
I track my sleep nightly to help nudge me to do the behaviors necessary to get the sleep I need. Here is a screenshot of my last year of sleep data, indicating that I put my money where my mouth is.
This is a term I created to describe your ideal sleep duration. When trying to determine how much sleep you need, you also must understand that your own sleep needs can change from day to day, depending on factors like fighting an infection, recovering from a higher-than-normal level of physical activity or stress, and more. So, even if you were to determine a number you felt was representative of your ideal, the reality is that your sleep need can change night to night. My recommendation is the same: allow for more time in bed than sleep you need. If you need eight hours of time in bed, plan to spend 8.5 hours in bed so you can wake under your own volition and not by an alarm clock. If your body needs a bit more sleep that night, you’ll have planned for this to happen. Give your body the chance to get all the sleep that it wants to get, and you will be doing yourself a favor. Wake up on your own volition and not by external means. It’s fine to set an alarm as a stopgap but you should aim, on average, to wake naturally.
Sleep science is complex, but sleep guidance doesn’t have to be. Good sleep is extraordinarily important for your health and daily performance. My view is that the reality of the modern world requires you to counteract its forces that impair health, and this is very true for sleep. Try the recommendations covered here and leave a comment reporting your findings after you’ve had the chance to let your experiment run for a few weeks.
This article does not address the all contingencies—especially sleep disorders and other conditions that disrupt sleep—that can alter what I discussed above. Additionally, the actual science is far more complex than what I presented here, but the message this article conveys, and the guidance it confers, is useful to help you view sleep in a healthy way, and ultimately, to maintain a healthy sleep practice throughout life.
About Dan: Dan is an entrepreneur and researcher whose life’s work is centered on how to facilitate healthy behaviors in others. He is the CEO of humanOS.me and dansplan.com, health technology companies that utilize the Loop Model to Sustain Health Behaviors—which he developed—to help people live a healthy lifestyle in a modern world (See Stanford MedX presentation on the Loop Model.) He does research with the Psychiatry and Behavioral Sciences Department at Stanford and the Departments of Neurology and Endocrinology at Leiden University in the Netherlands. His current research looks at how sleep influences decision-making (current list of publications). Dan also works with Naval Special Warfare to help the most elite fighters in the world maintain vigilant performance in both combat and non-combat conditions. Follow him @dansplanhealth.