“Circadian rhythm”. My biology teacher imposed me to write it 100 times. For sleeping in his class. He said, “Never forget that word. You are the best example for it”. The class nerds laughed at it. The rest including me did not understand the humor. Neither its meaning. Even after writing it 100 times!
Reading a time management topic last week, the knowledge unlocked. Circadian rhythm coordinates our sleep/wake cycle. This balance is maintained in the usual 24 Hours span. If you need a use case of it. Jet lag is a disruption of this rhythm.
Let us come back to the topic. The topic is not Circadian rhythm!
It is a shorter and similar Bio cycle called Ultradian rhythm. If Circadian rhythm is about 24Hrs cycle, Ultradian rhythm is shorter: 90 Minutes.
Ultradian rhythm coordinates our focus/distraction cycle. The balance is comprised in a 90 +20-minute span. And why am I taking the trouble of writing about it. Based on a little research I landed a reference of this concept in ancient Indian scriptures. Based on its History I presume this theory has been identified in earlier Indian civilization. Decades before Western lifestyle introduced it. This concept was defined in Chanakya’s Arthashastra.
So let me breakdown this into 2 parts. The Western Science Part & The Indian Shastra Part
The Western science of 90 Minutes.
Ultradian Rhythm = 90 Mins of High Frequency brain activity + 20 Minutes of Low Frequency Brain activity. Supported by the Sodium & Potassium level.
When we are alert and attentive, the brain is in a high frequency state. Gradually after more work the sodium/potassium balance is disrupted, and your brain moves to low frequency state You lose concertation and feel drained. This depletion mode normally happens after 90 Minutes.
That is why modern work life gurus suggests a break after 90 minutes . A 10-20 Minute break is sufficient to regain that Sodium / Potassium balance. So now you understand why productivity gurus ramble about 20 minutes power nap.
The Indian Shastra of 90 Minutes
In Contemporary system one Nadika is 90 minutes. Any task you do had to be comprised inside one Nadika and broken before starting the next 
Chanakya advises that a king should follow a time pattern in his daily life. A method suggested of dividing day and nights into 8 parts by means of nadikas . A day of 16 Nadikas.
Here is the Math: 1 Day = 24 Hours = 1440 Minutes = 16 Nadikas x 90 Minutes.
So to conclude this 90 minutes theory or the ultradian rhythm was an age old concept. With modern day implementations. That is why you have most foreign films in 90 Minutes. And some are three hours. The break comes in 90 minutes.
Here is the simple rule. Whatever essential task you are doing. Cut off after 90 minutes. Give some time for the brain to regain that focus. To replenish the Sodium and potassium. This can help you build a laser focus.
1 Inc.com Article on 90 minutes: https://www.inc.com/wanda-thibodeaux/why-working-in-90-minute-intervals-is-powerful-for-your-body-and-job-according-t.html
2 Tony Schwartz study of 90 Minutes: https://hbr.org/2011/01/the-most-important-practice-i
4 Chanakya In Daily Life – Radhakrishna Pillai, Page 58. Sutra# 1.19.6
Translation: “He should divide the Day & night in to eight parts, also the night by means of nadikas”.
Author: Arun Soman
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