The Growing Phase of Hair

Sleeping girl with long hair
Q: Someone told me that hair only grows at night when you are sleeping. Is this true?
A: No. This is not true. Your hair that is in its growing phase grows consistently throughout the 24-hour day. Of course, the hair of the average person only grows about half an inch per month (12.5 millimeters) which amounts to one-sixtieth of an inch (or .42 millimeters) per day. This is of course too little progress to really discern with the naked eye.
According to Dr. Mike Philpott of the Centre for Cutaneous Research of Barts and the London Queen Mary's School of Medicine and Dentistry, "There is evidence," Philpott says, "that hair growth follows a circadian [24-hour] rhythm." "We have in fact found some of the circadian clock genes are expressed in hair follicles."
It should be remembered that only 85-90% of the hair's follicles are in a growing phase at any given time. The remaining hairs are in a resting or shedding phase and are slated to be replaced when these phases complete. It's the duration of these cycles that determines the ultimate length our hair can achieve and at what density it remains throughout its length.
Photo: Denys Kurbatov/Shutterstock
See also:
The hair growth phases
What is hair made of and how does it grow?
Hair myths
How to protect your hair while you sleep