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How the Cycles of Light and Darkness Affect Your Health and Wellbeing

Story at-a-glance -

  • If you don’t sleep well, you’re not going to be optimally healthy no matter how good your diet and exercise are
  • Maintaining a natural rhythm of exposure to daylight, and darkness at night, is an essential component of sleeping well
  • Light is important because it serves as the major synchronizer of something called your master clock. Other biological clocks throughout your body in turn synchronize to your master clock
  • To maintain and “anchor” your master clock, you want to get bright outdoor light exposure for 30-60 minutes a day, ideally at solar noon. Sleep in maximal darkness. Blackout shades or a sleep mask is recommended
  • In the evening, avoid the blue light wavelength. This can be done by using blue-blocking light bulbs, dimming your lights, and if using a computer, installing a blue light-blocking software

By Dr. Mercola

While it may not be immediately obvious as to why, light is actually crucial to your health. I've always believed that you could have the ideal lifestyle with respect to the food you're eating, the water you're drinking, and exercise, but if you don't sleep well, you're just not going to be optimally healthy.

Poor sleep inevitably leads to health problems. Maintaining a natural rhythm of exposure to daylight, and darkness at night, is an essential component of sleeping well.

Researcher Dan Pardi works with the Behavioral Sciences Department at Stanford University and the Departments of Neurology and Endocrinology at Leiden University in the Netherlands.

"I look at how sleep deprivation, or not getting enough sleep or the amount of sleep that you need, can influence decision making and cognitive processes like reaction time, memory, impulsivity, and how that relates to food choice," he explains.

He's also the CEO of a health-behavior technology company called Dan's Plan, which seeks to help people optimize their health by establishing and sustaining an effective daily health practice, which includes maintaining good sleep habits.

How Much Sleep Do You Really Need?

Your sleep requirements change across your lifespan. Most infants will sleep a good percentage of the day. By adulthood, the amount of sleep typically settles somewhere around seven to nine hours. According to Pardi, the average, across a sample population, is about eight hours of sleep per night.

That said, sleep requirements are highly individual, and can change from one day to the next, depending on factors like stress, physical exertion, illness, pregnancy, and so on.

With so many variables, figuring out your optimal sleep requirement is a bit like a moving target. So, how can you tell you've achieved enough sleep? According to Pardi, the following three factors are key to determining how restorative sleep is:

  1. Duration— The number of hours you sleep
  2. Timing—i.e. going to bed at approximately the same time each night. Even if the duration of sleep is the same, when the timing of your sleep is shifted, it's not going to be as restorative
  3. Intensity—This has to do with the different stages that your brain and body goes through over the course of the night; the sequence of them, and how those stages are linked. Some medications will actually suppress certain phases of sleep, and certain conditions like sleep apnea will lead to fragmented sleep. With these scenarios, even if a person is sleeping for an adequate duration and has with consistent timing, sleep is not as restorative

One of the easiest ways to gauge whether you've slept enough is to assess your level of sleepiness the next day. For example, if you had the opportunity, would you be able to take a nap? Do you need caffeine to keep you going? That said, research has shown daytime sleepiness alone is not a complete way to assess the impairments of sleep loss.

According to Pardi:

"The reason why is that after a few days of insufficient sleep or not getting enough sleep on a daily basis, the amount of sleepiness that you experience tends to saturate. It doesn't get much worse; it gets pretty bad over a few days, and then it actually levels out," he explains.

"If you were to measure objective measures of cognitive performance like reaction time, you would see that those would continue to get worse if you maintained an insufficient sleep pattern on a day-by-day basis. That's one of the big problems in our society today. It's that we basically accommodate feeling sleepy that that feeling starts to feel normal. But then we have continual impairment in how well our brain is performing.

Your mind requires proper sleep to be able to focus its attention on one particular task... That is one of the cardinal aspects of attention deficit disorder that can happen to both adults and children.

We're getting 20 percent less sleep on average as a population than we were in 1960s. That's equivalent to one whole night of sleep loss, which is also clinically meaningful, meaning, at that level of sleep loss we experience very significant issues."

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How to Improve the Quality of Your Sleep

As mentioned earlier, the quality of your sleep has a lot to do with light, both outdoor and indoor lighting. The reason why light is important is because it serves as the major synchronizer of something called your master clock. This master clock is a group of cells in your brain called the suprachiasmatic nuclei (SCN). As a group, these nuclei synchronize to the light-dark cycle of your environment when light enters the eye. You also have other biological clocks throughout your body, and those clocks subsequently synchronize to your master clock.

"There are two levels of synchronizations taking place: the master clock with the environment and your body clocks with the master clock," Pardi explains. "So many different physiological factors will maintain a 24-hour pattern; whether that is eating behaviors, cell cycle, growth and repair process, or whether it's hormone patterns."

In the non-artificial light environment of our historical past, people experienced greater light exposure only during the day between when the sun rose and when it set. Now with the advent of the light bulb, artificial light, high-definition televisions, and any number of lighted electronic gadgets, we're exposed to a lot more light over a 24-hour period, and a lot less darkness. This creates a very novel situation for you internal time keeping and biological pace setting mechanisms of the body; in other words, your circadian rhythms. There are significant consequences to this, including but not limited to:

  • Alteration in behavior cognition, and mood
  • Alteration in physiological and cellular functions

Shift workers, for example, are known to have four- to five-folds higher rates of cancer than the average population. They also have higher rates of obesity and diabetes. But you may not have to be a classic shift worker to increase your risk for these conditions. As explained by Pardi:

"There's a new type of shift work, where people go to work in the morning, they come home, they spend a little bit of time with their families, and then on some nights they go back to work on their computers until late at night... This is a pattern that many people in the modern workplace maintain. Again, this new type of shift work significantly alters our internal rhythms because on some nights your getting light exposure for hours later than the day before. This variability in light instructs your rhythm-setting centers to constantly change body rhythms to try to catch up to the new pattern."

Exposure to Outdoor Light Is Critical for Maintaining Your Master Clock

Most people in Western societies spend the larger portion of each day indoors, which essentially puts you in a state of "light deficiency." In terms of light intensity, outdoor light is far more intense than indoor light. Light intensity is measured in lux units, and on any given day, the outdoor lux units will be around 100,000 at noon. Indoors, the typical average is somewhere between 100 to 2,000 lux units—basically two orders of magnitude less.

"We're not getting enough bright light exposure during the day, and then in the evening, we're getting too much artificial light exposure. Both of those have the consequence of causing our rhythms to get out of sync," Pardi says.

So, in terms of practical advice to help you maintain healthy master clock timing, you want to get bright light exposure during the day. Many indoor environments simply aren't intense enough to do the job well. So-called "anchor light" anchors your rhythm, causing it to be less fragile, so that light at night has less of an ability to shift your rhythm. As for how much light exposure you need, Pardi says the first 30-60 minutes of outdoor light exposure creates about 80 percent of the anchoring effect.

This is useful information indeed, as this means that even just going outside for half an hour at lunch time can provide you with the majority of anchoring light you need to maintain a healthy circadian rhythm. The ideal time to go outdoors is right around solar noon but any time during daylight hours is useful.

Exposure to Light Before Bedtime Hinders Sleep

Among other things, melatonin acts as a marker of your circadian phase or biological timing. In a nutshell, this hormone influences what time of day or night your body thinks it is, regardless of what time the clock on the wall displays. Somewhere between 50-1,000 lux is the activation range within which light will begin to suppress melatonin production. Melatonin is a regulator of your sleep cycle, and when it is suppressed, there is less stimulation to promote sleepiness at a healthy bedtime. This contributes to people staying up later and missing valuable sleep!

One 2011 study1 compared daily melatonin profiles in individuals living in room light (<200 lux) vs. dim light (<3 lux). Results showed that, compared with dim light, exposure to room light before bedtime suppressed melatonin in 99 percent of individuals, and shortened the time period when the body has an elevated melatonin level by about 90 minutes. Furthermore, exposure to room light during the usual hours of sleep suppressed melatonin by more than 50 percent. The authors concluded that

"These findings indicate that room light exerts a profound suppressive effect on melatonin levels and shortens the body's internal representation of night duration. Hence, chronically exposing oneself to electrical lighting in the late evening disrupts melatonin signaling and could therefore potentially impact sleep, thermoregulation, blood pressure, and glucose homeostasis."

Simply closing your eyes is not enough as light can penetrate your eyelids. Aim to make sure your bedroom is very dark. I recommend installing blackout shades for this purpose. A far less expensive alternative is to use a sleep mask to avoid disrupting your melatonin production and circadian rhythm. Also keep in mind that digital alarm clocks with blue light displays could have a detrimental effect.

The Right Type of Indoor Lighting Can Counteract Effects of Evening Light Exposure

The five lighting factors that affect your circadian rhythm are:

  1. Timing
  2. Intensity
  3. Duration of exposure
  4. History (how much light you got earlier in the day)
  5. The wavelength of the light

When it comes to the wavelength of light, you want to make sure you're using the right tone of light in the evening. The ideal light tone for any clock you keep on all night is a reddish amber, certainly not blue or green. The red and amber will interfere least with your melatonin production. You also want to simply keep your lights as dim as possible, so investing in dimmer switches is a good idea.

You can also address this issue on your computer. Pardi recommends a free computer program called f.lux (see justgetflux.com2), which alters the color temperature of your computer screen as the day goes on, pulling out the blue wavelengths as it gets later. According to the site: the program "makes the color of your computer's display adapt to the time of day, warm at night and like sunlight during the day." There are also blue light-blocking glasses and special blue light-blocking bulbs you can buy. Pardi explains why blocking blue light works:

"Most of your listeners have probably heard of rods and cones in the eye. These are specialized cells that can transduce a photo signal into a nerve signal. But with those cells, that signal will go back to the primary visual cortex where we can turn light into images that we can understand and see.

In the mid-90s, a different type of cell was discovered... [called] intrinsically photosensitive Retinal Ganglion Cells (ipRGC). It does the same thing as rods and cones: it transduced light to a nerve signal. But instead of the signal going to your visual cortex, it goes to your master clock. Those cells are most responsive to blue light. If you can block blue light, you can actually create something called circadian darkness or virtual darkness. What that means is that you can see, but your brain doesn't think that it's daytime; your brain thinks that it's in darkness.

That is actually a practical solution for living with artificial light in our modern world... With more awareness, future digital devices will adjust lighting in the evening to automatically dim and emit amber/red light [instead of blue]. This is much better for healthy circadian rhythms and sleep quality."

Other Lifestyle Variables That Affect Your Circadian Rhythm

Besides exposing yourself to intense outdoor light during the day for a minimum of 30 minutes, other variables may influence internal rhythms, especially the timing of food intake:

"For example, eating late at night is something that can decouple something called food-entrainable oscillator (FEO) from your master clock. Basically, your feeding patterns are in a rhythm that is orchestrated by the master clock. But you can decouple those rhythms by eating at a time that you usually don't eat – eating late at night," Pardi says.

He agrees that intermittent fasting is promising in terms of its beneficial health effects. However, he stresses the importance of not eating too late. Ideally, you want to avoid eating at least three hours prior to bedtime. What you eat in the evening can also be a factor that keeps you from falling into restful sleep.

"There are different strategies about when to fast. Not eating breakfast is one. The risk to that is that you then have more hunger late at night. That can be problematic actually. You could completely negate the potential positive effects by eating too late. Another strategy being explored by researcher Jeff Rothschild, is to just eat during daylight hours. You would eat in the morning, but you just wouldn't eat after darkness. If you do skip breakfast, I would also make it a firm rule to not eat within three hours of bed, and to keep you bedtime regular. Additionally, if you do this, be extra aware to get bright light exposure first thing on the morning"

For Optimal Health, Keep Your Master Clock Timed Right

In short, if you want to get good sleep, you have to have properly aligned circadian rhythms. If you don't, the varying aspects of your waking/sleeping system will be working at the wrong time. Insomnia is a common side effect of an improperly timed circadian clock. You'll also end up being sleepy during the day. Worse yet, it can have a significantly detrimental effect on your brain function and ability to perform at school and work.

So first and foremost, you want to maintain natural and proper light rhythms. This includes exposing yourself to intense light (aka daylight), ideally around solar noon but anytime during sun rise and sun down is useful, for at least half an hour or more each day. A gadget that can be helpful in instances when you, for some reason, cannot get outside during the day is a blue-light emitter. Philips makes one called goLITE BLU. (You can find it on Amazon3 for less than $150.) It's a small light therapy device you can keep on your desk. It's especially useful during winter when it's harder to get out door and there is a shorted photo period during the day (i.e., less daylight hours and less light intensity during daytime). Use it twice a day for about 15 minutes to help you anchor your circadian rhythm if going outdoors is challenging.

Then, in the evening, you want to dim environmental lights and avoid the blue light wavelength. Use blue-blocking light bulbs, dimming your lights with dimmer switches and turn off unneeded lights, and if using a computer, installing blue light-blocking software like f.lux.4 Last but not least, when it's time to go to sleep, make sure your bedroom is very dark.

Tracking Sleep to Maintain Mindfulness of Your Sleep Practice

As discussed, getting the right light exposure across the day, evening, and night is crucial to helping you get regular deep sleep and to support robust wakefulness during the day. It takes time to experience the maximal benefit of proper light exposure. You need to have the right light at the right time for multiple days in a row to experience the full effect. However, as we introduced in the beginning, duration and timing of sleep also impact sleep quality and daytime performance. In our modern world – due to a large amount of forces of the modern life Pardi says, it's easy to both get less sleep than you need and to have too much variability in when you sleep.

To solve this problem, he created a free sleep tracking tool (video description) on his website ( that uses effective behavioral techniques to keep you mindful of how you're living day by day. Making this sort of tool a part of your daily routine can lead to the addition of 30 extra minutes of sleep per night. If you're like most people, and you're getting insufficient sleep on a regular basis, these 30 minutes per night are a huge benefit. Practiced over time, the difference is equivalent to you missing 22 complete nights of sleep over one year!

"There are several new devices that provide feedback on sleep quality. However, it's normal for sleep to adjust itself every night so this sort of feedback – unless your diagnosing a sleep issue – isn't really necessary. At worst, it's misleading. A better use of these new technologies should aim to help you maintain the behaviors that help you get good sleep, like getting into bed at the right time. If I were to tell you that your sleep efficiency score from last night was 85 percent, what does that mean to you? Is that good, bad, or normal?

On the other hand, if the tool were to remind you that your target bedtime is, let's say, 10:45p, but you're going to bed on average at 11:30p recently, now you have increased mindfulness and a clear goal for what you can do tonight to get the sleep you need. That's very useful, especially since there are many temptations that make missing sleep easy.

This sort of tool helps you fight back, making the right sleep behavior more visible and salient in your day-to-day lifestyle. Tracking, therefore, is useful for both the novice and expert alike, because regardless of you level of knowledge of the sleep science, mindfulness of your own daily sleep practice helps you maintain a healthy pattern long term, and that's what counts in the end."

Bottom Line:

It's challenging to get the sleep you need in the modern world. To get the sleep that helps keep you healthy and performing at your best, Pardi recommends that it's useful to learn the fundamental components of good sleep (discussed here), maintain smart light rhythms day by day, and engage with the right tools to keep you mindful of your daily sleep practice. Sleep is hugely important in our health and these are some of the cutting-edge but practical techniques to help you get the best sleep possible.

You can also use the free software Dan compiled at as it allows you to easily track and integrate many of his suggestions.

For even more guidelines to help you get a good night's sleep, please see this previous article.