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How light exposure affects health – an interview of Dan by Dr. Joseph Mercola


Today, my video interview with Dr. Mercola was published. We talked about how to get great sleep and the influence of light on health, sleep, and daytime alertness.

Here are some of the summary notes posted on his website.

  • 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
  • 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 (F.Lux)


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 on improved daytime alertness. However, as discussed in the beginning of the interview, duration and timing of sleep also impact the equation. In our modern world – due to a large amount of forces of modern life – it’s easy to get less sleep than you need and to have too much variability in timing of your sleep.

At the end of the interview, we discussed self tracking for sleep practice mindfulness. To solve the problem of variable sleep timing and insufficient sleep duration, Dan’s Plan created a free sleep tracking tool (video description) that uses effective behavioral techniques to keep you mindful of how you’re living day by day. It’s not hard to imaging how making this tool a part of a person’s daily routine could 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 1 year!

Some new quantified self devices provide feedback on sleep stages. However, it’s normal for sleep to adjust night after night so this sort of detailed sleep analysis – 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%, 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, it’s useful to learn the fundamental components of good sleep (discussed in the video above), 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. If you’re not tracking sleep now, you should start today. Over the course of time, doing so will mean you’ll be more likely to get better sleep which will have enumerable benefits on your health and daily experience.


  • Vicki

    Hi Dan, recently listened to you talk to Amy on the Autoimmune Summit. I regularly go out for a lunch time walk but always wear sunglasses. Does wearing sunglasses reduce the benefits?

    • danpardi

      Hi Vicki. I stopped wearing glasses outside a while ago. I wear them only when needed during the day, like for example, making it easier to see when driving. I also wear light yellow // amber shade glasses in the evening about 2 hours from bed, especially when using technology products.

  • Vicki

    Thanks Dan. Not sure I can do that as I have light blue eyes and the light in Australia is very bright. But if I wear glasses will it negate the benefit of being out in the light? If it does I will try without but I’ll be squinting 🙂

    • danpardi

      Sure thing! Sun glasses reduce the amount of photons entering the eye. Being outside with glasses, depending on the shade strength of the lens, might be better than being indoors for circadian rhythm anchoring. I’d investigate finding a pair of glasses that have a light shade, and I would also aim to wear them outside as little as possible, depending on your comfort. That’s what I do.

      • Colin

        Hi Dan, love your work. Can you comment on the effect of wearing contact lenses that have uv blocking technology?

        • danpardi

          Thanks @disqus_wdu11NguWV:disqus! I don’t know much about these lenses but unless they block the transmittance of blue light, which I’m going to guess they don’t, then I don’t think it will have an affect on circadian rhythms. Cataracts, on the other hand, do (yellow the lenses in some cases), which may be one reasons older people have shifted circadian rhythms. This is being looked at currently.

  • Billius_53

    Dan. Great stuff. Curious is not full spectrum UV suppose to be harmful to the eyes, thus the recommendation to wear sunglasses by MDs for prevention of chronic eye problems? I’d gladly not wear sunglasses if I didn’t think they were providing any protection. Or is there cost/benefit trade off required with respect to chronic disease. For example: no sunglasses better sleep lower risk of chronic disease but higher risk of chronic eye disease or wear sunglasses sleep is not as good, but lowered risk of chronic eye disease, but greater risk of non-eye related chronic disease. Your thoughts please.

    • danpardi

      Hi @billius_53:disqus, it’s a good question. I do wear sunglasses in these situations: 1) driving when the sun is in my eyes, 2) if I’m outside a lot in one day. I try to make sure I get at least 30 minutes without sunglasses, and then after that, I choose to wear or not wear based on comfort and availability of my glasses. I’ll tell you this, my eyes used to be much more sensitive to sunlight when I wore glasses all the time. I rarely feel like I need glasses now, only when sun in right in my eyes while driving. Then they are helpful.

      • Billius_53

        I don’t think I ever wore sunglasses until I started driving. That was in my 1st summer of high school. I spent ever summer outdoors playing or in teens on the water canoeing and Sprint racing canoes. Sunglasses seemed more ubiquitous after high school. Worn outside except while playing sports. I now sail a lot, and have for 30 years and I get my best sleep when active outdoors. I always thought it was just the activity not the light, but when you are on the water even with sunglasses light comes to the eyes from the sides of glasses as well with reflection off the water.

        Fascinating topic. There appears to be lots of room for research here. Challenging research I suspect being difficult to get good controls?