Although many commercial products claim to feed or nourish hair with oils, herbs and fruit extracts, hair is unable to receive outside nutrition once it's grown past the scalp. That's because the hair on your head is technically dead. However, the hair cells beneath the scalp are still living and able to receive nutrients from the blood. Every individual hair has a life cycle, the process by which it goes from receiving nutrients to gracing your head to shedding.
Hair Root Structure
The base of each hair is housed in a little chamber called a follicle. At the very bottom of the follicle, the dermal papilla brings nutrients to the hair via the blood stream. Next to the hair shaft, the sebaceous gland produces an oily substance called sebum, which is your scalp's natural conditioner.
The first and longest part of your hair's life cycle is called the anogen phase, and about 85 percent of the hair on your head will be in this phase at any given time. This is the growing phase---hair grows about 1/2 inch every month---and it lasts for three to seven years. During this time, your hair is receiving nutrients from the dermal papilla.
After the growth cycle, hair takes a short transition period known as the catagen phase. During this time, which lasts for about two to four weeks, the blood supply to your hair will be slowly cut off, and your hair bulb---the bottom part of each strand of hair---will be pushed toward the surface of your scalp. About 2 to 3 percent of your total hair is in the catagen phase at any given time.
The telogen, or resting, phase is the final step of the cycle before your hair falls out. About 10 to 15 percent of the hair on your scalp is in the telogen phase. Hair does not grow, but about 25 to 100 hairs in this phase are shed daily. Your hair follicle is at rest and is not producing any new hair. After two to three months, the follicle reactivates, a new hair starts to grow, and the anogen phase begins all over again.
The growth and repair cycle of hair is random, not dependent on the seasons or any other external force. Unless you're experiencing unusual hair loss from medication, stress, pregnancy, diet or genetics, you shouldn't notice any difference in the hair on your head as the cycles progress. This is because the hair sheds evenly from your entire scalp, and the regrowth is also even, meaning you won't wake up with bald patches or sudden areas of new growth.