The Growing Phase of Hair
A: No, this is not true. Hair does not grow only at night or only while you are sleeping. Hair that is in the active growing phase grows continuously throughout the entire 24-hour day.
That said, there is evidence that hair follicles follow their own circadian (24-hour) biological rhythms. Studies indicate that skin cells in the root of the hair divide most rapidly (a process called mitosis) during late-night and early-morning hours while you are in deep sleep. This nighttime surge is fueled by increased blood flow and higher levels of human growth hormone release during deep sleep.
Conversely, other cellular processes predominate during waking hours. While hair growth might speed up slightly during rest, it never stops entirely. Growth continues around the clock. The body clock simply manages different upkeep tasks at different times of the day.
At any given time, approximately 85 to 90% of scalp hair follicles are in the active growing phase, known as the anagen phase. The other follicles are in a transition phase (catagen phase, which lasts 2 to 3 weeks) or a resting phase (telogen phase, lasting 2 to 3 months) before the hair sheds (exogen phase) to make room for a new strand.
The total duration of the active growing phase - usually lasting anywhere from 2 to 7 years, and largely determined by genetics - is the main factor that determines how long your hair can possibly grow.
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See also:
The hair growth phases
What is hair made of and how does it grow?
How to protect your hair while you sleep