Heri: Understanding your biological clock

Synchronizing with our biological clock

Biological Human Clock

The 24-hour light–dark cycle is a fundamental characteristic of Earth’s environment and profoundly influences the behaviour and physiology of animals and humans.

The illustration above highlight how hormone production and vital signs fluctuate during the day. This is called the circadian rhythm.

And it can be an important tool to help you schedule tasks and daily routine!

For example, cortisol production is at its highest in the morning. It lets us wake up, be alert and start our day. Cortisol then slowly decreases throughout the day. It’s at its lowest during the night to let us sleep and repair us physically and psychologically.

Melatonin secretion starts when our retina senses less light, and will stop after sunrise. The inner retina is especially sensitive to the lack of blue light wavelengths. Studies show for example that blue light will increase our alertness, performance and positive mood during the day. On the other hand, blocking blue light 3 hours before sleep significantly improves sleep quality.

Besides light, circadian rhythm is also sensitive to temperature. Lower temperatures in the evening will decrease alertness, metabolism as well as heart rate.

Other factors include meal times, stress and exercise. Weight training at 8pm can delay optimal sleeping time by 2 to 4 hours.

It is important to understand this biological human clock. Review the illustration above and look how you can (re)schedule work, nutrition, exercise and sleep. Your body will respond better, and your performance can improve, as well as your overall mood, health and well-being.

For example, if you usually sleep around 10:30pm, it is a good idea to stay away from 7:30pm blue light emitting devices such as iPads, mobile phones, TVs and most light bulbs. Studies show that you don’t have to be an active user: just having them in your bedroom is enough to show an effect. Of course, using actively an iPad or computer screen will increase stimulation, as well as increased potential stress from important emails and notifications.


Here are a few take-aways from the illustration and recent studies:

  • Optimal time to wake up is around 7 to 7:30am, when melatonin production stops
  • 8am or earlier is a good time for breakfast, for best ingestion of carbs.
  • Testosterone production is at its highest at 9am. A few will schedule their weight training for best results. But statistically, this is also a peak time for heart attacks or strokes. Adjust accordingly if you have hypertension or at risk for CVD.
  • Schedule your most important work between 10:00am and 11:30. This is where you are the most alert, good memory, and best ability to focus. Work in an area with plenty of blue natural light, as well as comfortable temperature. Lack of blue light such as working in a window-less office, or working in a cold environment will make you less productive and more frustrated.
  • Noon to 2:30pm is a dead time. Adrenal glandes and hormones are their lowest, and lunch digestion will drag you down. Have one cup of coffee (or better yet healthier coffee alternatives), and schedule light work, such as emails or follow-ups.
  • 2:30pm to 6pm is good for more active work. Go meet and talk to colleagues.
  • 5pm is the best time for cardiovascular activities. Your lungs will be most efficient around that time. Take your Garmin or Suunto GPS sports watch, and hit the road!
  • 6:30pm to 7:30pm is when your blood pressure and temperature are at their highest. Don’t be stuck in traffic or have a heated emotional argument at that time!
  • Avoid carbs at dinner. Pasta, rice, bread will be transformed quickly into fat if ingested in the evening. Salad with a bit of your favourite protein is a better choice.
  • Stay away blue light 3 hours before you sleep. Exposure to amber light can help on the other hand.
  • night

If you work in night shifts, the best investment is a portable blue light for work (such as this) as well as a pair of blue light blocking glasses when you are going to sleep, to minimize circadian rhythm disruption. However, asking to take day shifts instead is better for your health.

In case there is daylight savings, shift circadian times above so timing syncs with your local sunrise time.


  • Antoine U Viola, Lynette M James, Luc JM Schlangen and Derk-Jan Dijk. Blue-enriched white light in the workplace improves self-reported alertness, performance and sleep quality. Scandinavian Journal of Work, Environment & Health.  Vol. 34, No. 4 (August 2008), pp. 297-306
  • Dijk D-J, Archer SN (2009) Light, Sleep, and Circadian Rhythms: Together Again. PLoS Biol 7(6): e1000145. doi:10.1371/journal.pbio.1000145

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