ESSENTIAL Health Considerations on Polyphasic Schedules: The ULTIMATE Course on the Dark Period

The Nervous System: An In-depth Explanation Why Certain Activities Should be Avoided

The Nervous System: An In-depth Explanation Why Certain Activities Should be Avoided

This section will stress on:

  • The autonomic nervous system and its relationship with the dark period
  • Why you should not do certain activities during the dark period. 



The autonomic nervous system contains two subparts, the Sympathetic Nervous System (SNS) and the Parasympathetic Nervous System (PNS).

  • These nervous systems correlate to the circadian rhythm.
  1. The SNS is dominant during the day
  2. The PNS is dominant during the night.
  • In terms of functions:
  1. The SNS is responsible for your fight-or-flight system.
  2. The PNS is responsible for the secretion of sleep-related hormones. Its roles include recovering the body and the mind, de-stressing and much more.
  • SNS activation increases blood pressure and heart rate. It also stimulates the medulla glandulae suprarenalis and secretes adrenaline and noradrenaline.
  • An active SNS inhibits digestion activities.
  • PNS activation, on the other hand, raises peristalsis and secretion of digestive hormones. In addition, it also decreases heart rate and blood pressure.

After a quick glance, it is clear that these two parts of the autonomic nervous system are responsible for completely different bodily functions.

Optimal circadian health

The optimal circadian health is achievable by allowing the SNS to be dominant during the day and not during the night. This is because constantly keeping the SNS engaged will elevate blood pressures. This increases the chance of developing various diseases mentioned below. Engaging the SNS during the night will also lead to:

  • More stress
  • Hypertension
  • Ischemic heart disease
  • Chronic heart failure
  • Kidney disease
  • Type II diabetes
  • Obesity,
  • Obstructive sleep apnea,
  • Depression and various other diseases1.

In other words, overworking the SNS leads to the potential development of stress-related disorders.

Controlling SNS activation

Since activating the SNS around the clock is a bad idea, you need to know how to avoid doing it. So, how do you activate the SNS? Well, there are several ways to do it.

  • Activities like eating and exercising activates the SNS. Thus, it means that you should avoid these both during the dark period and close to your habitual sleep times. This is because only either nervous system can be active at once. Thus, if you engage the SNS, your PNS will deactivate.

Let’s use an example to help ground this point.

  • Activating the PNS is similar to doing a relaxing task, like stretching or meditating.
  • Activating the SNS, on the other hand, shares some grounds with a physically strenuous task like running or exercising intensely.

It is only possible to do either one of these at once; this means that activating one of the systems will decrease the activity of the other one significantly. In this example, you cannot really stretch while running; the situation is very similar to the actual bodily functions.

As opposed to activating the SNS, activating the PNS around the clock does not have anything to do with any negative effects. On the contrary, since activating the PNS reduces blood pressure, it is very healthy in the middle of the day. This can facilitate taking a short daytime nap for example.

Immediate consequences

Apart from the long-term consequences of keeping the SNS active around the clock, there are also immediate effects.

  • If you are trying to sleep after engaging the SNS, you will have trouble falling asleep, issues staying asleep. Your sleep quality will also suffer.
  • In other words, when you engage your SNS, it will take longer to fall asleep with increased heart rate and adrenaline secretion. You will also have a harder time staying asleep and may experience intermittent wakes.
  • When you finally wake up, you will not feel rested because you have missed out on some valuable vital sleep.

Community recommendations

Because of this, these are the following recommendations regarding activities that activate your SNS:


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Sample non-eating window on Triphasic
  • Upon eating large meals, their effects last up to 2 hours on elevated muscle sympathetic nerve activity and blood pressure. Both of these will negatively affect your sleep quality and the duration of vital sleep stages.
  • Avoid large meals 2 to 3 hours before a sleep session. The muscle sympathetic nerve activity and the increased blood pressure also lead to the suppression of your PNS; thus, you should also avoid eating during the dark period.
  • Small meals of no more than ~150 kcal should not have any negative effect on your sleep. Therefore, you should be safe to consume that amount before a nap without any downsides.


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Sample non-exercising window on Everyman 1
  • When you exercise, your sympathetic activity will increase. Because of that, you may not be able to fully cool down during your dark period.
  • If you wish to exercise before a scheduled sleep time, consider its intensity carefully. More intensity leads to a longer necessary winding down time of up to 2 hours.
  • Because of the increased sympathetic activity, you should also avoid exercising during the dark period.

Drinking water

  • You should drink water throughout the night to stay hydrated. However, you should avoid drinking lots of fluids at once.
  • The healthiest option is to drink around a glass of water every hour during the night.


  1. Fisher, J. P., Young, C. N., & Fadel, P. J. (2009). Central sympathetic overactivity: maladies and mechanisms. Autonomic Neuroscience148(1-2), 5-15.

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2 thoughts on “ESSENTIAL Health Considerations on Polyphasic Schedules: The ULTIMATE Course on the Dark Period

  1. Hmm, I wonder why it says ‘Enroll’ for course & a lock symbol even though I clicked on enroll

    1. You would need to create an account on the website to view everything. If you don’t want to do so, you can see the abbreviated version in the Overview tab.

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