Since I was young, I've associated being clean with taking a daily shower. I still 100% believe showering daily is true. But I no longer believe washing hair everyday is a healthy practice. In fact, it is unnecessary and probably unhealthy for your hair and scalp. First, let's go over 3 reasons why it's unhealthy to wash your hair daily.
Washing hair everyday removes sebum
Sebum is natural hair oil that moisturizes and waterproofs skin and hair. So washing out Sebum everyday dries out your skin and causes prematurely shedding. In Western culture, we dislike oily hair which means daily hair washing.
Shampoo Contains Chemical Sulfates
Shampoos contain sulfates that give the "bubbling" effect. The "bubbling" effect is what we equate with cleanliness. Unfortunately, the sulfates are not necessary for cleansing the scalp. "Sodium lauryl sulfate" (SLS) is contained in over 90% of shampoos, which in long usages conditions is a skin irritant.
Sweat is Water Soluble
Because sweat is water soluble, rinsing your hair with only water (without shampoo) will dissolve sweat into the water and flushed out of your hair and scalp. It however is not effective at removing dirt and oil (sebum).
So these three facts have sparked a no 'poo' movement, where people are washing hair with shampoo less often and use sulfate-free shampoo. I've heard stories of people washing their hair once every four days. I've also been told a good way to determine when to use shampoo is when your hair becomes unmanageably oily.
Given the facts, I've decided to give myself the challenge of fewer hair washes with shampoo, but instead only rinse with water. Given that I bike to work, and morning and afternoon activity with a shower right after each, I figure I'm on the extreme end of the challenge.